This protocol is intended to let special clients create transient-for
relationships between X11 and Wayland windows. The client that needs
this is xdg-desktop-portal-gnome, which will create e.g. file chooser
Wayland dialogs that should be mapped on top of X11 windows.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2810>
