Implement ConnectionStatus class. Fix online / offline mode issues.

[dero]

Summary

Fixes #85
Fixes #94
Fixes #97

This PR introduces a few changes to the UI and refactors the way offline / online connection status is determined, reported and signaled.

There are a few notable changes, some of which deviate from the current state of things – those attempt to address the UX discrepancies reported under #85. Please review these changes one by one and LMK if you feel something needs to change. With the new underlying architecture in place, changes should be relatively simple.

Toggle button in the header now reads "Offline Mode" and its purpose is to toggle Offline Mode on and off.

Offline Mode button will refuse to deactivate when client is offline. Instead it will display a "Not connected" banner.

2021-03-16_18-53-02.mp4

There is no dangling banner on the screen indicating whether client is online or offline.

The "Show offline content only" button in settings now has the function as the Offline Mode button.

2021-03-16_18-56-42.mp4

Checklist

• My pull request is addressing an open issue (please create one otherwise).
• My code is tested and passes existing tests.
• My code follows the Contributing Guidelines (updates are often made to the guidelines, check it out periodically).

[dero]

@derekherman Updated per our call this morning.

1. Removed the Offline mode toggle from the menu.
2. Re-styled the "No connection" message + it's now displayed the whole time the client is offline.
3. When the user tried to exit "Offline mode" and the client is offline, the "No connection" message shakes to indicate the intended action is not possible at the moment.

Screencast showing the offline message and the alert shake.

2021-03-17_10-20-56.mp4