kiosk-shell: Store kiosk_shell_output in kiosk_shell_surface

We end up doing some extra list walks and mildly awkward contortions to
find the kiosk_shell_output from the weston_output.

Store the kiosk_shell_output instead - it's always trivial to find the
weston_output from that, and it just seems a bit cleaner conceptually for
kiosk shell abstractions to point to other kiosk shell abstractions when
available.

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