kiosk-shell, xwayland: Add a new weston_xwayland_surface_api function

Such that shells can retrieve Xwayland's surface WM_CLASS/WM_NAME and use
it to place the corresponding Xwayland surface on the appropriate
output, similar to what xdg_shell::set_app_id does.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>