Add a new lock screen menu to combine volume network and power

The design has a combined volume-network-power indicator in the lock
screen, which when opened shows a volume slider. Implement it by abstracting
the volume menu into a PopupMenuSection, and by creating three StIcons
bound to the real ones.

https://bugzilla.gnome.org/show_bug.cgi?id=682540

js/ui/panelMenu.js

@@ -236,7 +236,8 @@ const SystemStatusButton = new Lang.Class({
         this._box = new St.BoxLayout({ style_class: 'panel-status-button-box' });
         this.actor.add_actor(this._box);
 
-        this.setIcon(iconName);
+        if (iconName)
+            this.setIcon(iconName);
     },
 
     addIcon: function(gicon) {