window.c: Don't assume registry advertisement order

The toytoolkit assumes that wl_seats are advertised after
wl_data_device_manager and creates a data_device during wl_seat
registry binding. This patch removes this assumption by creating
data_devices for all the wl_seats created up until then.

Fixes: https://gitlab.freedesktop.org/wayland/weston/issues/201



Signed-off-by: Harish Krupo <harishkrupo@gmail.com>
(cherry picked from commit ee4c7a24)