GTK 4.12 deprecates gdk_wayland_toplevel_unexport_handle() in favor
of the new gdk_wayland_toplevel_drop_exported_handle(). We are not
bound by API stability, so we can just expose the additional argument
that the replacement requires instead of tracking the handle
internally.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2778>
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
