view: added :event-activated signal

Since the events widget are handled entirely inside GcalView
implementations (aka the views), we need a way to hook event-widget
activation with GcalWindow which is handling the dialogs (as expected).
This signal serve as a bridge between those two objects.

src/gcal-view.h

@@ -46,6 +46,7 @@ struct _GcalViewIface
 
   /* signals */
   void            (*create_event)                       (GcalView *view, icaltimetype *start_span, icaltimetype *end_span, gdouble x, gdouble y);
+  void            (*event_activated)                    (GcalView *view, GcalEventWidget *event_widget);
 
 /* FIXME remove me in favor of the one below */
   void            (*clear_selection)                    (GcalView *view);