Use basic GtkBox widget for headerbar replacement

Window headerbar is hidden at all times. We do not want to execute the logic that comes with GtkHeaderBar, so we simplify it by using GtkBox as a dummy widget.

src/app.js

@@ -30,7 +30,7 @@ class ClapperApp extends AppBase
         const clapperWidget = new Widget();
         window.set_child(clapperWidget);
 
-        const dummyHeaderbar = new Gtk.HeaderBar({
+        const dummyHeaderbar = new Gtk.Box({
             can_focus: false,
             focusable: false,
             visible: false,