main: Activate notification proxy on start

At least for the time being, this looks like the easiest option to
launch the service:

 - we could add a systemd unit, but then we'd need to update the
   RequiredComponents in the fallback session definition as well,
   making it necessary for gnome-shell, gnome-shell-extensions and
   gnome-session to be updated to 3.36.1 in lockstep

 - autostart is problematic as it would make gnome-shell conflict
   with other notification daemons; also autostart is most useful
   with automatic shutdown, which would require tracking signal
   subscriber to determine when the service is unused

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

js/ui/main.js

@@ -142,6 +142,9 @@ function start() {
     shellDBusService = new ShellDBus.GnomeShell();
     shellMountOpDBusService = new ShellMountOperation.GnomeShellMountOpHandler();
 
+    Gio.DBus.session.watch_name('org.gnome.Shell.Notifications',
+        Gio.BusNameWatcherFlags.AUTO_START, null, null);
+
     _sessionUpdated();
 }