While the gsetting is available for all who needs it, the Shell might
override it given various hueristics. Expose the decision made by the
Shell via a new property.
Intended to be used by gsd-xsettings as well as xdg-desktop-portal-gtk.
This also add a version property to the API, so that semi external
services (xdg-desktop-portal-gtk) can detect what API is expected to be
present.
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/757
9ab0071aa5