appMenu: Hide stopped spinner actor

Get rid of leftover empty space from the application menu panel
button, that was used by the spinner actor, which remained visible
even after the spinner had stopped.

Fixes: https://gitlab.gnome.org/GNOME/gnome-shell/issues/1679
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/832

js/ui/panel.js

@@ -239,6 +239,7 @@ var AppMenuButton = GObject.registerClass({
 
         this._spinner = new Animation.Spinner(PANEL_ICON_SIZE, {
             animate: true,
+            hideOnStop: true,
         });
         this._container.add_actor(this._spinner);