altTab: Also return EVENT_PROPAGATE in override of _onItemMotion()

Clutter expects a return value here, so make sure we also return
Clutter.EVENT_PROPAGATE just like the function we override does.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2097>

js/ui/altTab.js

@@ -814,6 +814,8 @@ class AppSwitcher extends SwitcherPopup.SwitcherList {
         } else {
             this._itemEntered(index);
         }
+
+        return Clutter.EVENT_PROPAGATE;
     }
 
     _enterItem(index) {