X11 applications expect -geometry command line option to work for
setting the initial window position, but currently this does not work.

Add provision to relay the initial position through libweston-desktop:
- weston_desktop_api gains a new entry set_xwayland_position
- implement set_toplevel_with_position() in xwayland internal interface

Once xwayland plugin starts calling set_toplevel_with_position(),
libweston-desktop can relay that information to a shell if the shell
wants to hear it.

If a shell does not implement the set_xwayland_position hook, the
old behaviour remains: the shell positions xwayland toplevels like any
other.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Reviewed-by: Daniel Stone <daniels@collabora.com>