extensions-app: Explicitly dispose window when removed

Unlike in previous GTK version (or Clutter), destroy() no longer
breaks reference cycles but just releases the reference held by
GTK itself.

So any reference we hold - either the explicit property or any signal
handlers that bind the window as `this` - prevents the window from
being disposed when closed, and the application won't quit.

Work around this by explicitly running dispose() on the window when
it is removed from the application.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1495>

subprojects/extensions-app/js/main.js

@@ -42,6 +42,8 @@ class Application extends Gtk.Application {
     _init() {
         GLib.set_prgname('gnome-extensions-app');
         super._init({ application_id: 'org.gnome.Extensions' });
+
+        this.connect('window-removed', (a, window) => window.run_dispose());
     }
 
     get shellProxy() {