[xmonad] QuickCheck properties for layouts

I just pushed a patch fully testing, via QuickCheck properties, the builtin tall layout, and how it handles messages. That code is fully covered now. To layout authors, using pureLayout and pureMessage, have a poke around Properties.hs , to get an idea how to generate random window manager states, apply your layouts to them, and then to test their behaviour with random message events. -- Don (xmonad testing czar)