Fixed the invalidation method.
Implemented (f)locking in Core\Cache\Files
Fixed the *PropelCacheObject in Core\Kryn
Invalidate the *PropelCacheObject cache if a propel objects is updated
Removed 'return;' in ka.Desktop, that prevents the icons to be loaded.
Store a backup in ClientAbstract in getUser() if we got a user.
Adjusted the return value in postDesktop in Admin\Backend
Added a test that proves that cache invalidation works.