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.

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

closes #41

closes #52

The mxid input will not prevent the user from entering any invalid
value.

closes #22

closes #11

- Adds detection for duplicate events

closes #46