The X-Resource extension client library (XRes) allows a client to query
the Xserver for various resources associated with an X11 client,
including its PID.

Add (optional) support for XRes in xfwm4 and use it for getting the
client's PID instead of relying on _NET_WM_PID.

Signed-off-by: Olivier Fourdan <fourdan@xfce.org>