Implement snap packaging, snaps are universal Linux packages(#704)

[Snaps][1] are universal Linux packages that can be installed and run on
many GNU+Linux distributions with ease.

This patch implements the necessary details to package
SimpleScreenRecorder as a snap.  [Install Snapcraft][2] and run
`snapcraft` under the source tree to build it.

[1]: https://snapcraft.io
[2]: https://docs.snapcraft.io/snapcraft-overview/8940

Signed-off-by: 林博仁(Buo-ren Lin) <Buo.Ren.Lin@gmail.com>

.gitignore

@@ -25,3 +25,12 @@ build-*
 src/simplescreenrecorder
 
 private-*
+
+# Snap packaging specific rules
+/snap/.snapcraft/
+/parts/
+/stage/
+/prime/
+
+/*.snap
+/*_source.tar.bz2

snap/local/launchers/simplescreenrecorder-launch

@@ -0,0 +1,14 @@
+#!/usr/bin/env bash
+# This is the maintainence launcher for the snap, make necessary runtime environment changes to make the snap work here.  You may also insert security confinement/deprecation/obsoletion notice of the snap here.
+
+set \
+	-o errexit \
+	-o errtrace \
+	-o nounset \
+	-o pipefail
+
+# gtk-common-themes support
+export QT_QPA_PLATFORMTHEME=gtk3
+
+# Finally run the next part of the command chain
+exec "${@}"

snap/local/patching/patch-desktop-entries.bash

@@ -0,0 +1,217 @@
+#!/usr/bin/env bash
+# Utility script to patch desktop entries in snap, should only be run in the `override-prime` scriptlet
+# 林博仁 © 2018
+
+## Makes debuggers' life easier - Unofficial Bash Strict Mode
+## BASHDOC: Shell Builtin Commands - Modifying Shell Behavior - The Set Builtin
+set \
+	-o errexit \
+	-o errtrace \
+	-o nounset \
+	-o pipefail
+
+## Runtime Dependencies Checking
+declare\
+	runtime_dependency_checking_result=still-pass\
+	required_software
+
+for required_command in \
+	basename \
+	dirname \
+	realpath; do
+	if ! command -v "${required_command}" &>/dev/null; then
+		runtime_dependency_checking_result=fail
+
+		case "${required_command}" in
+			basename \
+			|dirname \
+			|realpath)
+				required_software='GNU Coreutils'
+				;;
+			find \
+			|xargs)
+				required_software='GNU Findutils'
+				;;
+			sed)
+				required_software='GNU Sed'
+				;;
+			*)
+				required_software="${required_command}"
+				;;
+		esac
+
+		printf -- \
+			'Error: This program requires "%s" to be installed and its executables in the executable searching paths.\n' \
+			"${required_software}" \
+			1>&2
+		unset required_software
+	fi
+done; unset required_command required_software
+
+if [ "${runtime_dependency_checking_result}" = fail ]; then
+	printf -- \
+		'Error: Runtime dependency checking fail, the progrom cannot continue.\n' \
+		1>&2
+	exit 1
+fi; unset runtime_dependency_checking_result
+
+## Non-overridable Primitive Variables
+## BASHDOC: Shell Variables » Bash Variables
+## BASHDOC: Basic Shell Features » Shell Parameters » Special Parameters
+if [ -v 'BASH_SOURCE[0]' ]; then
+	RUNTIME_EXECUTABLE_PATH="$(realpath --strip "${BASH_SOURCE[0]}")"
+	RUNTIME_EXECUTABLE_FILENAME="$(basename "${RUNTIME_EXECUTABLE_PATH}")"
+	RUNTIME_EXECUTABLE_NAME="${RUNTIME_EXECUTABLE_FILENAME%.*}"
+	RUNTIME_EXECUTABLE_DIRECTORY="$(dirname "${RUNTIME_EXECUTABLE_PATH}")"
+	RUNTIME_COMMANDLINE_BASECOMMAND="${0}"
+	# We intentionally leaves these variables for script developers
+	# shellcheck disable=SC2034
+	declare -r \
+		RUNTIME_EXECUTABLE_PATH \
+		RUNTIME_EXECUTABLE_FILENAME \
+		RUNTIME_EXECUTABLE_NAME \
+		RUNTIME_EXECUTABLE_DIRECTORY \
+		RUNTIME_COMMANDLINE_BASECOMMAND
+fi
+declare -ar RUNTIME_COMMANDLINE_ARGUMENTS=("${@}")
+
+## init function: entrypoint of main program
+## This function is called near the end of the file,
+## with the script's command-line parameters as arguments
+init(){
+	if ! process_commandline_arguments; then
+		printf -- \
+			'Error: Invalid command-line parameters.\n' \
+			1>&2
+
+		printf '\n' # separate error message and help message
+		print_help
+		exit 1
+	fi
+
+	if ! test -v SNAPCRAFT_PRIME \
+		|| test "${PWD}" != "${SNAPCRAFT_PRIME}"; then
+		printf -- \
+			"%s: Error: This script should be run by \`snapcraft\` in the \`override-prime\` scriptlet.\\n" \
+			"${RUNTIME_EXECUTABLE_NAME}"
+		exit 1
+	fi
+
+	if test -d share/applications; then
+		find \
+			share/applications \
+			-name '*.desktop' \
+			-print0 \
+			| xargs \
+			--no-run-if-empty \
+			--null \
+			--verbose \
+			sed \
+			--file "${RUNTIME_EXECUTABLE_DIRECTORY}"/patch-desktop-entries.sed \
+			--in-place
+	fi
+
+	exit 0
+}; declare -fr init
+
+print_help(){
+	# Backticks in help message is Markdown's <code> markup
+	# shellcheck disable=SC2016
+	{
+		printf '# Help Information for %s #\n' \
+			"${RUNTIME_COMMANDLINE_BASECOMMAND}"
+		printf '## SYNOPSIS ##\n'
+		printf '* `"%s" _command-line_options_`\n\n' \
+			"${RUNTIME_COMMANDLINE_BASECOMMAND}"
+
+		printf '## COMMAND-LINE OPTIONS ##\n'
+		printf '### `-d` / `--debug` ###\n'
+		printf 'Enable script debugging\n\n'
+
+		printf '### `-h` / `--help` ###\n'
+		printf 'Print this message\n\n'
+	}
+	return 0
+}; declare -fr print_help;
+
+process_commandline_arguments() {
+	if [ "${#RUNTIME_COMMANDLINE_ARGUMENTS[@]}" -eq 0 ]; then
+		return 0
+	fi
+
+	# Modifyable parameters for parsing by consuming
+	local -a parameters=("${RUNTIME_COMMANDLINE_ARGUMENTS[@]}")
+
+	# Normally we won't want debug traces to appear during parameter parsing, so we add this flag and defer its activation till returning(Y: Do debug)
+	local enable_debug=N
+
+	while true; do
+		if [ "${#parameters[@]}" -eq 0 ]; then
+			break
+		else
+			case "${parameters[0]}" in
+				--debug \
+				|-d)
+					enable_debug=Y
+					;;
+				--help \
+				|-h)
+					print_help;
+					exit 0
+					;;
+				*)
+					printf -- \
+						'%s: Error: Unknown command-line argument "%s"\n' \
+						"${FUNCNAME[0]}" \
+						"${parameters[0]}" \
+						>&2
+					return 1
+					;;
+			esac
+			# shift array by 1 = unset 1st then repack
+			unset 'parameters[0]'
+			if [ "${#parameters[@]}" -ne 0 ]; then
+				parameters=("${parameters[@]}")
+			fi
+		fi
+	done
+
+	if [ "${enable_debug}" = Y ]; then
+		trap 'trap_return "${FUNCNAME[0]}"' RETURN
+		set -o xtrace
+	fi
+	return 0
+}; declare -fr process_commandline_arguments
+
+## Traps: Functions that are triggered when certain condition occurred
+## Shell Builtin Commands » Bourne Shell Builtins » trap
+trap_errexit(){
+	printf \
+		'An error occurred and the script is prematurely aborted\n' \
+		1>&2
+	return 0
+}; declare -fr trap_errexit; trap trap_errexit ERR
+
+trap_exit(){
+	return 0
+}; declare -fr trap_exit; trap trap_exit EXIT
+
+trap_return(){
+	local returning_function="${1}"
+
+	printf \
+		'DEBUG: %s: returning from %s\n' \
+		"${FUNCNAME[0]}" \
+		"${returning_function}" \
+		1>&2
+}; declare -fr trap_return
+
+trap_interrupt(){
+	printf '\n' # Separate previous output
+	printf \
+		'Recieved SIGINT, script is interrupted.' \
+		1>&2
+	return 1
+}; declare -fr trap_interrupt; trap trap_interrupt INT
+
+init "${@}"

snap/local/patching/patch-desktop-entries.sed

@@ -0,0 +1,28 @@
+# This sed script patches the desktop entries of the snapped
+# application.
+#
+# Documentation:
+#
+# * GNU Sed Manual
+#   https://www.gnu.org/software/sed/manual
+#     * `sed` script overview - `sed` scripts
+#     * `sed` commands summary - `sed` scripts
+#     * The `s` Command - `sed` scripts
+#     * Overview of basic regular expression syntax - Regular
+#       Expressions: selecting text
+#     * Back-references and Subexpressions - Regular Expressions:
+#       selecting text
+
+## Append '(Snap)' to the application name to make it
+## distinguishable with the other same application using different
+## software distribution technologies
+##
+## FIXME: The appended string is not localizable, the proper way to
+##        implement this is to probably use a new X-Snappy-Name keys
+##        with localestring format to let the translators fill in
+##        additional localized string and use these values to replace
+##        the Name keys here.
+s/^\(Name\(\[.\+\]\)\?=.*\)$/\1 (Snap)/g
+
+## Fix-up application icon lookup
+s|^Icon=.*|Icon=\${SNAP}/meta/gui/icon.svg|

snap/local/scriptlets/main-adopt-info

@@ -0,0 +1,74 @@
+#!/usr/bin/env bash
+# Extract and set the snap's version from the main part
+# 林博仁(Buo-ren, Lin) <Buo.Ren.Lin@gmail.com> © 2019
+
+set \
+    -o errexit \
+    -o errtrace \
+    -o nounset \
+    -o pipefail
+
+init(){
+    local \
+        flag_dry_run=false \
+        upstream_version \
+        packaging_revision \
+        some_place_under_the_project_repo \
+        snap_version_string
+
+    for commandline_argument in "${@}"; do
+        case "${commandline_argument}" in
+            # Enable execution tracing
+            --debug)
+                set -o xtrace
+            ;;
+            # Don't run snapcraftctl for testing purpose
+            --dry-run)
+                flag_dry_run=true
+            ;;
+            --usage)
+                print_usage
+                exit 0
+            ;;
+            *)
+                printf -- \
+                    'main-adopt-info: Error: Invalid command-line argument.\n' \
+                    >&2
+                print_usage
+                exit 1
+            ;;
+        esac
+    done
+
+    if ! git describe &>/dev/null; then
+        upstream_version=unknown
+    else
+        upstream_version="$(
+            git \
+                describe \
+                --always \
+                --dirty=-d \
+                --tags \
+            | sed s/^v//
+        )"
+    fi
+
+    snap_version_string="${upstream_version}"
+
+    printf -- \
+        'main-adopt-info: Setting snap version to "%s".\n' \
+        "${snap_version_string}"
+    if [ "${flag_dry_run}" = false ]; then
+        snapcraftctl set-version \
+            "${snap_version_string}"
+    fi
+
+    exit 0
+}
+
+print_usage(){
+    printf -- 'Usage: main-adopt-info [options] _snap_name_\n'
+    printf -- 'options: [--debug|--dry-run|--help]\n'
+}
+
+init "${@}"