appDisplay: Add the folder popup to AllView itself

Now the the folder popup behaves like a dialog, it must be
above the app grid, and not be affected by the scroll view
translation.

Add the folder popup to the AllView itself, instead of the
internal Shell.Stack that is inside the scroll view.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/896

js/ui/appDisplay.js

@@ -703,7 +703,7 @@ var AllView = GObject.registerClass({
     }
 
     addFolderPopup(popup) {
-        this._stack.add_actor(popup);
+        this.add_child(popup);
         popup.connect('open-state-changed', (o, isOpen) => {
             this._eventBlocker.visible = isOpen;