[Xmonad] darcs patch: add new LayoutHelpers module.

[Donald Bruce Stewart]

droundy:
> Here's a start of a sketch of a helper module to make it easier to write
> correct layouts, and also to insulate layout-writing code from changes to
> the Layout representation in XMonad core (e.g. so we can make it safe to
> modify layouts in doLayout, as LayoutHooks and SimpleStacking do).

Good idea. Applied.

We always hoped a DSL for layouts would emerge.

-- Don