[Xmonad] The great xmonad-class merge!

[Spencer Janssen]

I've merged all the changes from the xmonad-class branch to the core.  There
are a few changes users and developers ought to be aware of.

Layout state (such as the currently selected layout, nmasters, the
master/slave ratio split, etc.) now persists across restarts.  Because the old
layout-resetting behavior of mod-q is sometimes useful, we've added
mod-shift-space to the keybindings to reset layout settings on the current
workspace.

Some contributed modules haven't been ported yet:
    HintedTile
    Magnifier
    Mosaic
    SimpleStacking
    Spiral
    SwitchTrans

Feel free to pitch in with the porting effort.

Developers will notice that we've moved from the record of functions approach
to a Layout class.  So far, this has been a big win, allowing us to write
layouts and layout transformers in a more structured fashion.  Porting simple
layouts to the new system is fairly easy, see the core layouts for examples.

Special thanks to David Roundy and Andrea Rossato for initiating this branch
and doing the lion's share of porting to the new system.


Cheers,
Spencer Janssen