lockScreenMenu: Construct actual panel icons much more accurately

Instead of faking it by adding a bunch of main icons and secondary
icons to our own box, try and recreate the original button box
with the original icons.

https://bugzilla.gnome.org/show_bug.cgi?id=690589

js/ui/panelMenu.js

@@ -244,11 +244,17 @@ const SystemStatusButton = new Lang.Class({
             this.setIcon(iconName);
     },
 
+    get icons() {
+        return this._box.get_children();
+    },
+
     addIcon: function(gicon) {
         let icon = new St.Icon({ gicon: gicon,
                                  style_class: 'system-status-icon' });
         this._box.add_actor(icon);
 
+        this.emit('icons-changed');
+
         return icon;
     },