Xwayland: Unprivileged rootless support via ext-pacement

wayland/wayland-protocols!247 (closed) provides an ext-placement protocol that can be used to approximate absolute window positioning. Unlike xwayland-shell, ext-placement is an unprivileged protocol and could be used to allow rootless Xwayland to operate without special privileges. Not all compositors will support ext-placement, but I expect that at least COSMIC will.