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.
_NET_STARTUP_ID helps to associate toplevels with startup-ids
this signals the end of the startup sequence.
(cherry picked from commit e479dc1e)