Added undo notification system to event deletion.

Added GcalEventView::will-delete to let thw window knows about the change.
GcalWindow will handle the hiding of the event-widget till the notification goes
away or the user click "Undo".
Added method in GcalView interface to retrieve and event-widget by it's uuid.
Added some cosmetic changes in GcalMonthView as well.

src/gcal-event-view.h

@@ -49,7 +49,7 @@ struct _GcalEventViewClass
   GtkGridClass parent_class;
 
   /* Signals */
-  void (*done) (GcalEventView *view);
+  void (*will_delete) (GcalEventView *view, const gchar* event_uuid);
 };
 
 GType         gcal_event_view_get_type                   (void);