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.

data/gtk-styles.css

@@ -57,7 +57,7 @@
   padding-right: 8px;
 }
 
-GtkClutterOffscreen.sources {
+GtkClutterOffscreen.overlay {
   background-color: transparent;
   background-image: none;
 }