Large refactoring of the Kicker backend with new features.

* Not just apps, also documents and contacts can now be added
  to the favorites. All types of content have their full context
  menu action list available also in the favorites sidebar.
* Contact favorites use the contact photo as icon, with a circular
  alpha mask and the presence status icon as overlay badge.
* Adding/removing favorites from context menus now works every-
  where, e.g. also in KRunner-based search results.
* Far fewer actions now cause a full rebuild of the model tree
  and instead only delta updates, as a side-effect of adding a
  mechanism for this to e.g. handle status changes for contact
  items.
* Various bugs around hiding/unhiding apps got fixed.
* Lots of code cleanup + less coupling all around.