[Xmonad] darcs patch: make layouts preserved over restart (and 1 more)

The new change here is a default modifyLayout. This implements the old idModify from LayoutHelpers, so that it's easy to create layouts that don't need to read messages. And it costs us no extra code in core, since Full falls in this category. :) David Fri Sep 21 16:43:16 EDT 2007 David Roundy * make layouts preserved over restart Sun Sep 23 07:52:19 EDT 2007 David Roundy * create default modifyLayout that ignores messages.