layout: Don't offset trayBox when panel is hidden

The panel is not visible when in fullscreen, but critical notifications
may still be shown - having them pop out from where the panel would be
looks unpolished, so adjust the trayBox accordingly.

https://bugzilla.gnome.org/show_bug.cgi?id=744850

js/ui/layout.js

@@ -287,6 +287,9 @@ const LayoutManager = new Lang.Class({
 
         let trayConstraint = new MonitorConstraint({ primary: true, work_area: true });
         this.trayBox.add_constraint(trayConstraint);
+        this.panelBox.bind_property('visible', trayConstraint, 'work-area',
+                                    GObject.BindingFlags.SYNC_CREATE);
+
 
         this._loadBackground();
     },