xwayland: Allow to retrieve startup-id via _NET_STARTUP_INFO

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

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