Allows one to check whether the server is currently ready.
This allows users to setup event listeners before the server is actually started.
This can be used to know when wlr_xwayland_server decides to start a new Xwayland process. At that point the wl_client has already been created but the Xwayland process hasn't been started yet.
We're about to get one more Xwayland-related thing, and this header already contains two things.