In order to support OpenExtensionPrefs()'s parentWindow parameter,
we will need the ability to make a window transient to an external
window identified by a string handle.
This takes the corresponding code from xdg-desktop-portal-gtk and
brings it into an introspectable form, likewise the counterpart in
libportal to export a string handle for a GtkWindow.
https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1087
