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

Basically reverts the last font related commits since pointSize isn't
as reliable as pixelSize.

Also some layout values (margins, spacings) have been moved out to Config.h.

* Validate both inferred and explicitly entered server addresses by attempting to call the /versions endpoint
* If the domain from the mxid fails validation, try prefixing it with 'matrix'
* Only show server address field if address validation ultimately fails