[xmonad] Decoration rewrite

[wagnerdm at seas.upenn.edu]

Quoting Gwern Branwen <gwern0 at gmail.com>:

> User-wise, I'm not concerned. There's only one config in my copy of
> the config archive which even imports XMonad.Layout.Decoration, and
> that config apparently isn't even current since it doesn't show up
> when I download a fresh copy*; perfectly acceptable collateral damage.

Given the amount of traffic on the list about decorations in the last  
few days, I'd take this as evidence that your config archive is  
woefully incomplete, not that decorations are unused by xmonad users.

~d