Notification backends now can emit a signal activated() that tells MainWin to raise...

For now, this signal is emitted by systray, dbus and knotify. Unfortunately, raise() does not seem
to work with kwin and (according to docs) neither on Windows. Furthermore, my knotify seems
to be broken and doesn't signal a click at all. Thus, this whole thing got "limited" testing by me :)

Take it as "might work under some circumstances in some environments". Feedback welcome.