main: Notify gnome-session when we're ready

main: Notify gnome-session when we're ready

Basically, we recently[1] removed XSMP from gnome-session because it's
been disabled for a while. Mutter also[2] removed its XSMP integration
shortly after.

However, it turns out that gnome-session's builtin service manager was
relying on Mutter's XSMP integration to know when gnome-shell is done
starting up. No more XSMP = no more notification = gnome-session decides
that gnome-shell is dead even though it's not = fail whale

This is a temporary measure. GDM will soon switch[3] to an architecture
that will allow us to completely drop gnome-session's builtin service
manager, and this workaround will go away

[1]: https://gitlab.gnome.org/GNOME/gnome-session/-/merge_requests/139
[2]: https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/4438
[3]: https://gitlab.gnome.org/GNOME/gdm/-/merge_requests/289

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3730>