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.
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.
