[Xmonad] darcs patch: refactor layout interface.

Hi all, Here's an attempt at an interface that allows the definition of custom workspace layouts in Config. I'm not really pleased with it, so you can view it as a discussion piece more than an actual proposal. In particular, it punts on the idea of any sort of mouse interaction with a layout, which almost makes a tabbed layout useless. Still, I think it's an interesting approach to allowing layouts that contain arbitrary sorts of state. David Mon Apr 16 20:48:13 PDT 2007 David Roundy * refactor layout interface. This change only changes functionality in that the tileFraction is now a per-layout variable. The main point is to add the flexibility to define layouts in the Config file. There's a bit of cheesiness in the type of modifyLayout, since I could figure out a pleasant choice for passing the key-binding information to the Layout, so I just used a string.