[xmonad] darcs patch: hide existential Layout (mostly) from user API.

Hi all, This allows us to eliminate the wart of requiring a Layout wrapper around our layoutHook. Besides being ugly, this wrapper hampers the composability of layouts: we can't simply write something like: myconfig = defaultConfig { layoutHook = Layout $ Mirror $ layoutHook defaultConfig } because there's no Read instance for Layout Window, so Mirror (Layout Window) has no Read instance. Obviously, we could get rid of this with an unLayout higher-order function, but I see no reason to expose our users to this at all. David Sat Nov 10 16:30:55 PST 2007 David Roundy * hide existential Layout (mostly) from user API.