6baf490aab
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