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/messageTray.js

@@ -2493,15 +2493,7 @@ const SystemNotificationSource = new Lang.Class({
     Extends: Source,
 
     _init: function() {
-        this.parent(_("System Information"));
-
-        this._setSummaryIcon(this.createNotificationIcon());
-    },
-
-    createNotificationIcon: function() {
-        return new St.Icon({ icon_name: 'dialog-information',
-                             icon_type: St.IconType.SYMBOLIC,
-                             icon_size: this.ICON_SIZE });
+        this.parent(_("System Information"), 'dialog-information', St.IconType.SYMBOLIC);
     },
 
     open: function() {