messageTray: Emit signal when notifications are enabled/disabled

Since the introduction of per-source notification policy in commit
098bd4509b, the NotificationPolicy::enable-changed signal has been
used to track the 'enable' setting. However as we never actually
emitted that signal, this never worked without a restart - oops.

https://bugzilla.gnome.org/show_bug.cgi?id=749279

js/ui/messageTray.js

@@ -258,6 +258,8 @@ const NotificationApplicationPolicy = new Lang.Class({
 
     _changed: function(settings, key) {
         this.emit('policy-changed', key);
+        if (key == 'enable')
+            this.emit('enable-changed');
     },
 
     _canonicalizeId: function(id) {