messageTray: Primarily use a GIcon to drive the source's icon

This is a bit of a cleanup since we ported notification icons/secondary
icons to be in the same situation.

https://bugzilla.gnome.org/show_bug.cgi?id=680426

js/ui/messageTray.js

@@ -1122,10 +1122,14 @@ const Source = new Lang.Class({
     // Provides a sane default implementation, override if you need
     // something more fancy.
     createIcon: function(size) {
-        return new St.Icon({ icon_name: this.iconName,
+        return new St.Icon({ gicon: this.getIcon(),
                              icon_size: size });
     },
 
+    getIcon: function() {
+        return new Gio.ThemedIcon({ name: this.iconName });
+    },
+
     _ensureMainIcon: function() {
         if (this._mainIcon)
             return;