Fix system tray, clean it up

At first I noticed it wasn't working like it should, then realised it was using APIs that are largely unadvised. Also I saw there's some kind of system to render numbers in there but this is also something we generally do not do. There's better ways to display such information (e.g. using the com.canonical.Unity interface), so I wouldn't bother having ad-hoc code in there. And if we need to have it, we better have it in KNotifications. It now will also use the icon from the theme.