[xmonad] QuickCheck properties for layouts

[Don Stewart]

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)