Handled sources_model::active changes.

Handled sources_model::active changes by emitting fake ::events-added and
::events-removed signals.
Updated methods hooked to ECalClientView signals to take into account
GcalManagerUnit::enabled field.

src/gcal-week-view.c

@@ -518,6 +518,7 @@ gcal_week_view_add (GtkContainer *container,
             gcal_event_widget_peek_uuid (GCAL_EVENT_WIDGET (widget)),
             gcal_event_widget_peek_uuid (GCAL_EVENT_WIDGET (child->widget))) == 0)
         {
+          //TODO: remove once the main-dev phase its over
           g_warning ("Trying to add an event with the same uuid to the view");
           return;
         }