clients/window: Defer closing of window

Instead of closing the window directly by calling close_handler() use a
deferred task to do that instead.

That way we avoid a potential invalid access on a link which was
previously removed, due to the fact both window_destroy() and
touch_handle_up() traverse the same list.

This is an alternative to 841.

Fixes: #607.

Suggested-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Reported-by: He Yong <hyyoxhk@163.com>
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>