xwayland: Introduce a private struct for XWayland interface

libweston-desktop implements this private struct.

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Reviewed-by: Jonas Ã…dahl <jadahl@gmail.com>
Acked-by: Giulio Camuffo <giulio.camuffo@kdab.com>

Differential Revision: https://phabricator.freedesktop.org/D1208
7 files changed