notificationDaemon: Save notifications on source destruction

While the existing comment is correct in that a source's notifications
will be destroyed first, the code takes a shortcut which prevents the
Source::count-updated signal from being emitted. Given that the purpose
of the signal is to keep notification counters up-to-date which is
pointless when the source is about to be destroyed, the shortcut makes
sense; just save notifications explicitly in that case.

https://bugzilla.gnome.org/show_bug.cgi?id=710596

js/ui/notificationDaemon.js

@@ -905,10 +905,8 @@ const GtkNotificationDaemon = new Lang.Class({
         let source = new GtkNotificationDaemonAppSource(appId);
 
         source.connect('destroy', Lang.bind(this, function() {
-            // When a source is destroyed, it should first
-            // destroy all of its notifications, so we should
-            // not need to call _saveNotifications here.
             delete this._sources[appId];
+            this._saveNotifications();
         }));
         source.connect('count-updated', Lang.bind(this, this._saveNotifications));
         Main.messageTray.add(source);