xwm: let the shells decide the position of X windows

xwm: let the shells decide the position of X windows

The xwm used to automatically send to Xwayland the position of X windows
when that changed, using the x,y of the primary view of the surface.
This works fine for the desktop shell but less so for others.
This patch adds a 'send_position' vfunc to the weston_shell_client that
the shell will call when it wants to let Xwayland know what the position
of a window is.
The logic used by the desktop-shell for that is exactly the same the xwm
used to have.
Reviewed-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: David Fort <contact@hardening-consulting.com>