Make it possible to install gz-sim without the GUI components

[j-rivero]

Currently there is no option of installing gz-sim without the GUI components. Motivated by osrf/docker_images#354 it should be possible to repacked the software to isolated the gui components in there own package and make then a soft dependency so no gui installations are possible.

[j-rivero]

Looking into the gz-sim code, the Qt dependencies seems to be encapsulated in src/gui/CMakeLists.txt although the Qt5 dependency appears directly on the main CMakeLists.txt.

Looking into the -release repository the gui component is being stored into the libgz-sim package. That needs to be split out into its own package, something libgz-sim-gui.

gz-sim8-release on  main ❯ grep -R gui 
ubuntu/debian/libgz-sim.install:usr/share/*/*-*[0-99]/gui/*