dbusServices/notifications: Add a separate notification daemon

Add a small service that exposes the Fdo notification API under the
well-known name, and forwards any requests to the actual implementation
in the shell.

That way any app with permission to talk to org.freedesktop.Notifications
will get exactly that, and nothing more.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/547

js/dbusServices/notifications/main.js

@@ -0,0 +1,11 @@
+/* exported main */
+
+const { DBusService } = imports.dbusService;
+const { NotificationDaemon } = imports.notificationDaemon;
+
+function main() {
+    const service = new DBusService(
+        'org.gnome.Shell.Notifications',
+        new NotificationDaemon());
+    service.run();
+}