messageTray: Notify count when adding a notification

Adding a notification clearly constitutes a count change, but
the notify call was accidentally dropped during the overhaul
of the notification API.

Fixes: 34f05b075b ("messageTray: Let the tray decide whether to show a
banner")

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3308>

js/ui/messageTray.js

@@ -572,6 +572,7 @@ export const Source = GObject.registerClass({
 
         this.emit('notification-added', notification);
         this.emit('notification-request-banner', notification);
+        this.countUpdated();
     }
 
     destroy(reason) {