EventModel signaling changed

 - if some of the data (favourite,alarm) has changed on the event,
   signal 'eventHasChanged' is emitted
 - all treeViews (eg. DayView, FavsView, TracksView, ...) have to listen on this signal
Only favouritiesView is 'reset' when current tab is changed in mainWindow
 - 'cause time groupings have to be recreated, since favs may have changed