_NET_ACTIVE_WINDOW: set client urgent, if not made visible

Set the client's urgent flag, if it could not be made visible via
client_focus and client_raise.

This is a good compromise: it does not steal focus, if the client is not
in the current tag set, but makes it easy to get to it.

Ref: https://awesome.naquadah.org/bugs/index.php?do=details&task_id=848