Improve systray notifications

* Add an action button to KDE's notification bubble, as it doesn't report
  clicks other than action triggers
* Bring Quassel to front on click (thanks Zarin for pointing out how to bypass focus
  stealing prevention)
* Select buffer where the highlight occurred on click

Note: Plasma notifications are mostly broken currently at least for me; YMMV. If it doesn't work for
you, try using old-fashioned notifications in KDE and see if it works there.