xwm: Prepare send_configurenotify for non-fullscreen use

Currently weston_wm_window_send_configurenotify is only called for
fullscreen clients, and it is written to be correct only in that case.

Fix it up to handle other cases properly so we can use it for them in a
later commit. Synthetic Configure Notify events are relative to the
root window, so this means adding our window co-ordinate when
necessary.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>