appDisplay: Start always at scroll 0 on FolderView

Reset the scroll adjustment between popups opennings,
following the same design we want to the AllView

https://bugzilla.gnome.org/show_bug.cgi?id=706081

js/ui/appDisplay.js

@@ -960,6 +960,7 @@ const FolderIcon = new Lang.Class({
             function() {
                 this._ensurePopup();
                 this._popup.toggle();
+                this.view.actor.vscroll.adjustment.value = 0;
             }));
         this.actor.connect('notify::mapped', Lang.bind(this,
             function() {