xmonad: reformat command strings using Data.List.intercalate

http://www.haskell.org/haskellwiki/Xmonad/Config_archive/Mntnoe%27s_xmonad.hs#lib.2FDMenu.hs
xmonad: clean up formatting
xmonad: fix single-screen layout for laptop

This still uses a hard-coded screen width (1280), which isn't ideal.
Maybe one day I'll learn enough Haskell to query this at runtime...