Make Cache constructor exception free

fixes #74

* Initial "join room" feature.
* React correctly to remotely joined rooms.
* Leaving rooms implemented both locally using the room menu
   in nheko, and reacting properly when leaving a room remotely 
   from another client.

Fixes #50.

On KDE desktop icon failed to appear because TrayIcon requested a zero-size rect. Implementing MsgCountComposedIcon::availableSizes() method fixes that.

After icon became visible it was not transparent, and places that should have been transparent contained artifacts likely due to uninitialized memory. Implementing MsgCountComposedIcon::pixmap() which returns a pixmap with alpha channel fixes that.

fixes #55

Add the packaging metadata to build the nheko snap

Silence errors from redacted events

matrix-org/synapse#2358 proposes a change to only contain objects
in the sync stream where values get updated.
The current behavior leads nheko to fail when that is used.

fixes #14

Ctrl-Q from the main window now quits the application.

fixes #61