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-window.h

@@ -58,6 +58,8 @@ GtkWidget*   gcal_window_new               (GcalApplication *app);
 void         gcal_window_show_notification (GcalWindow      *window,
                                             GtkWidget       *notification);
 
+void         gcal_window_hide_notification (GcalWindow      *window);
+
 G_END_DECLS
 
 #endif /* __GCAL_WINDOW_H__ */