Port message tray sources and notifications to class framework

Third step in the class framework port, now it's the turn of
MessageTray.Source and MessageTray.Notification, as well as
the various implementations around the shell.

https://bugzilla.gnome.org/show_bug.cgi?id=664436

js/ui/status/network.js

@@ -2121,15 +2121,12 @@ const NMApplet = new Lang.Class({
     }
 });
 
-function NMMessageTraySource() {
-    this._init();
-}
-
-NMMessageTraySource.prototype = {
-    __proto__: MessageTray.Source.prototype,
+const NMMessageTraySource = new Lang.Class({
+    Name: 'NMMessageTraySource',
+    Extends: MessageTray.Source,
 
     _init: function() {
-        MessageTray.Source.prototype._init.call(this, _("Network Manager"));
+        this.parent(_("Network Manager"));
 
         let icon = new St.Icon({ icon_name: 'network-transmit-receive',
                                  icon_type: St.IconType.SYMBOLIC,
@@ -2137,4 +2134,4 @@ NMMessageTraySource.prototype = {
                                });
         this._setSummaryIcon(icon);
     }
-};
+});