[xmonad] darcs patch: new workspace sorting function in TopicSpace

Attached is a patch which adds a new exported function to TopicSpace, appropriate for providing as the ppSort field of a PP DynamicLog record, which sorts workspaces by history. The current facilities provided by TopicSpace for interfacing with DynamicLog are much too monolithic; this patch adds functionality that I want (and which others might find useful) but it really ought to be completely refactored to provide many smaller components that users can assemble however they like. -Brent