xwayland: Allow to retrieve startup-id via _NET_STARTUP_INFO

Needed for https://gitlab.gnome.org/World/Phosh/phoc/-/merge_requests/259

Cherry-pick from wlroots master.

Original commit description:

A launchee notifies with a "remove"¹ message when done starting up. Catch these and forward to the compositor. This allows the compositor to end the startup sequence that might have been started by another protocol like xdg-activation.

We don't handle other messages since we expect the launcher to use a wayland protocol like xdg-activation.

While _NET_STARTUP_ID helps to associate toplevels with startup-ids this signals the end of the startup sequence.

1. https://specifications.freedesktop.org/startup-notification-spec/startup-notification-latest.txt

(cherry picked from commit e479dc1e)