[Xmonad] xmonad is a zipper

Hey all. Back from vacation, with a shiny new xmonad StackSet implementation based on a 'zipper'. The benefit: 50 lines less code (particularly, Operations.hs gets simpler by far) tracks focus automatically by design (no delete-transient focus-lost bug) I'll have a rather long blog article on the zipper type tomorrow. Spencer, can you have a look. I'd like to merge this in. Needs some merging with the changes during the last week. http://www.cse.unsw.edu.au/~dons/code/xmonad/StackSet.hs http://www.cse.unsw.edu.au/~dons/code/xmonad/Operations.hs -- Don