Mentions are now an '@' icon in the upper right.

UI Popup is now a smaller dialog.

Still lots of work to be done here.

The room list would be hidden only through window resizing.

Ctrl-O -> Chat
Ctrl-L -> Room list

fixes #251

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.

Fixes #95

* 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.

closes #52

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.

Logout from the current session and invalidate the current token