[Xmonad] xmonad is a zipper

[Donald Bruce Stewart]

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