Rootless XWayland unprivileged mode

Rootless XWayland currently requires support from the Wayland compositor and access to privileged XWayland-specific protocols. In some environments, such as sandboxed Flatpaks without X11 access, such support is not available. It is also possible that a compositor simply has not implemented XWayland support.

While supporting the full X11 feature set under rootless XWayland is not possible without compositor support, it is possible to support many X11 applications by using various heuristics. This was already implemented in https://github.com/talex5/wayland-proxy-virtwl, but would be nice if XWayland supported this natively.