A stable version of xdg shell has now been released.

https://cgit.freedesktop.org/wayland/wayland-protocols/plain/stable/xdg-shell/xdg-shell.xml

It's not very different from xdg-shell unstable v6, but it's not backwards compatible, so we need to create another shell extension for it.

It may have some naming conflicts with xdg-shell unstable v5, due to how qwaylandscanner works, but at least the global interface name is different ("xdg_wm_base"), so it should be possible to provide support for both at the same time.