osdWindow: Don't inherit from St.Widget

We don't apply any styling to the toplevel OSD actor, so we can just
as well avoid the style machinery and use a plain actor.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2127>

js/ui/osdWindow.js

@@ -42,7 +42,7 @@ class OsdWindowConstraint extends Clutter.Constraint {
 });
 
 var OsdWindow = GObject.registerClass(
-class OsdWindow extends St.Widget {
+class OsdWindow extends Clutter.Actor {
     _init(monitorIndex) {
         super._init({
             x_expand: true,