Experimental port of RecentDocsModel to KActivitiesStats.

Needs kactivities.git:ivan/libkactivities-experimental-stats

Known problems:
* The model class hierarchy in Kicker needs refactoring to
  work properly with proxy models; lots of mess right now
  just to get things to display for testing.
* Due to the above, the funnel model that enables proper
  delegate instance reuse when switching a menu dialog
  between sibling categories is currently disabled, causing
  bad performance.
* Forget/Forget All for documents isn't available anymore
  due to missing API in KActivities.
* KActivitiesStats' ResultModel isn't inserting rows when
  new files are opened at runtime.

Ivan: This currently assumes my locally hacked up KActivities
copy to fix the CMake problems; please feel free to push to
this branch when you've made the upstream CMake changes to
make it build.

CCMAIL:ivan.cukic@kde.org