Panel: move the _panelContainer down to PanelMenu

Panel already forces each item to be a PanelMenu.Button, so it's better
to have the latter handle the bin container too, instead of attaching
a private property that might collide with internal usage by the indicator.

https://bugzilla.gnome.org/show_bug.cgi?id=683156

js/ui/panelMenu.js

@@ -21,6 +21,10 @@ const ButtonBox = new Lang.Class({
         this.actor = new Shell.GenericContainer(params);
         this.actor._delegate = this;
 
+        this.container = new St.Bin({ y_fill: true,
+                                      x_fill: true,
+                                      child: this.actor });
+
         this.actor.connect('get-preferred-width', Lang.bind(this, this._getPreferredWidth));
         this.actor.connect('get-preferred-height', Lang.bind(this, this._getPreferredHeight));
         this.actor.connect('allocate', Lang.bind(this, this._allocate));