Handle _NET_CLOSE_WINDOW client message requests

> Pagers wanting to close a window MUST send a _NET_CLOSE_WINDOW client
> message request to the root window.

We interpret this message as a request to close the con for the given
window.

See: http://standards.freedesktop.org/wm-spec/wm-spec-latest.html#idm140200472668896

fixes #1396