appDisplay: Fix return value for _onScroll

ClutterActor::scroll-event has a boolean return value to indicate
whether the event has been handled, or event emission should continue.
Now that we are using an StScrollView, we depend on this to avoid
propagating the event to the view's own handler.

https://bugzilla.gnome.org/show_bug.cgi?id=707409

js/ui/appDisplay.js

@@ -402,7 +402,7 @@ const AllView = new Lang.Class({
 
     _onScroll: function(actor, event) {
         if (this._displayingPopup)
-            return;
+            return true;
         let direction = event.get_scroll_direction();
         if (direction == Clutter.ScrollDirection.UP) {
             if (this._currentPage > 0)
@@ -413,6 +413,7 @@ const AllView = new Lang.Class({
                     this.goToPage(this._currentPage + 1);
             }
         }
+        return true;
     },
 
     _onPan: function(action) {