Fixed bug caused by me forgetting what types im using (invites appearing at the bottom of the list instead of the top when priority sorting was off)

When user is mentioned (via matrix 'highlight_count'), inactive
rooms will use a different color for the notification circle than
when only general unread messages exist.

errors

Update dark and system themes to provide
colors for the hover states for title, subtitle, and timestamp.

Add room list hover title/subtitle color change support.

fixes #313

fixes #330

fixes #2

fixes #157

fixes #128

The color scheme of nheko obeys the default color theme of Qt
(i.e. the system theme). It uses a Qt stylesheet to accomplish this,
which means replacing the color theme with a custom theme would only
be a matter of writing a new style sheet and loading it into the app.

* 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.