Start using MessageTray.Source directly instead of having to subclass it

For most subclasses, this is a direct swap -- a lot of the time, the
constructor was a blank class that override createNotificationIcon,
and called _setSummaryIcon in _init.

https://bugzilla.gnome.org/show_bug.cgi?id=661236

js/ui/windowAttentionHandler.js

@@ -53,10 +53,10 @@ const Source = new Lang.Class({
     Extends: MessageTray.Source,
 
     _init: function(app, window) {
-        this.parent(app.get_name());
         this._window = window;
         this._app = app;
-        this._setSummaryIcon(this.createNotificationIcon());
+
+        this.parent(app.get_name());
 
         this.signalIDs = [];
         this.signalIDs.push(this._window.connect('notify::demands-attention', Lang.bind(this, function() { this.destroy(); })));