Untieing needs to be done prior to the destruction of the object which
aggregated the TiedPropertyList. So the TiedPropertyLists destructor can't
untie the properties automatically (and it doesn't). So, do some manual
clean-up.
Priority scheme for tile loading/removing
Cleaner tile manager interface for AI/groundcache/...
Reduce start-up delay. Drop splash screen when inner scenery is available.
Be nice to osg loader: slow-down main loop while initial scenery is still loading.
