appDisplay: Fix app folder "modal-ness"

Commit 5d00c1a5ee moved app folder popups to GrabHelper - for some
reason, the line that ensures the current behavior of only considering
events inside the app picker to dismiss popups got lost ...

js/ui/appDisplay.js

@@ -1251,6 +1251,7 @@ const AppFolderPopup = new Lang.Class({
                 this.actor.destroy();
             }));
         this._grabHelper = new GrabHelper.GrabHelper(this.actor);
+        this._grabHelper.addActor(Main.layoutManager.overviewGroup);
     },
 
     toggle: function() {