Draft: Add support for the XWAYLAND extension

This extension exists to serve one purpose: reliably identifying Xwayland. Previous attempts at doing so included querying root window properties, output names or input device names. All these attempts are somewhat unreliable. Instead, let's use an extension - where that extension is present we have an Xwayland server.

Clients should never need to do anything but check whether the extension exists through XQueryExtension or search through XListExtensions.

This extension provides a single QueryVersion request only, and that is only to provide future compatibility if we ever need anything other than "this extension exists" functionality.

See the corresponding xorgproto MR xorgproto!54 (merged)

Note: I'm filing this MR primarily so it's archived, not because I think it should be merged. As above, the extension just needs to exist, it doesn't actually need an implementation. I needed the implementation only to test the server.