[xmonad] swapScreen in cycleWS

Hi, I'm not sure why, but it seems darcs send didn't work, so i send the patch manually. It would be nice if someone could review it, though it is basically the same as switchScreen. I find this useful because often i want to swap the contents of my screens. If you think it is appropriate, please include it. Thanks, Tom -- Then I drew in a breath, and my renewed will with it, lifted the rod in my right hand, murmured a phrase in a language I didn't know, and blew the tires off his fucking truck. -- Harry Dresden