Don't mix GJS and GObject signal systems

GJS implements a basic signal system that allows monkey-patching
JS objects with signal methods resembling the GObject ones. However
it's clearly not a good idea to replace the actual GObject methods,
so use the proper GObject facilities when inheriting from GObject.

https://bugzilla.gnome.org/show_bug.cgi?id=778660

js/ui/appDisplay.js

@@ -899,6 +899,8 @@ const ControlsBoxLayout = Lang.Class({
 const ViewStackLayout = new Lang.Class({
     Name: 'ViewStackLayout',
     Extends: Clutter.BinLayout,
+    Signals: { 'allocated-size-changed': { param_types: [GObject.TYPE_INT,
+                                                         GObject.TYPE_INT] } },
 
     vfunc_allocate: function (actor, box, flags) {
         let availWidth = box.x2 - box.x1;
@@ -909,7 +911,6 @@ const ViewStackLayout = new Lang.Class({
         this.parent(actor, box, flags);
     }
 });
-Signals.addSignalMethods(ViewStackLayout.prototype);
 
 const AppDisplay = new Lang.Class({
     Name: 'AppDisplay',