xwayland: Add xwayland.h and move more things to xwayland.c

xwayland: Add xwayland.h and move more things to xwayland.c

- Move xwayland-specific struct definitions to new xwayland.h header
- Move xwayland_move_sub_views_to_front() from desktop.c to xwayland.c
- Split out xwayland_server_init/finish() from server_init/finish()
- Rename new_xwayland_surface -> xwayland_new_surface and
  xwayland_surface_new() -> handle_new_surface() for consistency
- Add "mapped" argument to xwayland_unmanaged_create() so that we can
  make unmanaged_handle_map() private to xwayland-unmanaged.c