Add and update the _NET_CLIENT_LIST property on the root window to
better comply with ewmh standards.
Information on this property can be found here:
http://standards.freedesktop.org/wm-spec/latest/ar01s03.html
> These arrays contain all X Windows managed by the Window Manager.
> _NET_CLIENT_LIST has initial mapping order, starting with the oldest window.
fixes#1099
Tony Crisci