xdg-shell: Put xdg_shell events after requests

It makes the structure consistent with most other protocols and
provides a clear separation between what is done by the server and what
is done by the client.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by: Mike Blumenkrantz <zmike@osg.samsung.com>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>