* Export WorkspaceCompare and WorkspaceSort types.
* Extract commonality in sort methods into mkWsSort, which creates
a workspace sort from a workspace comparison function.
* Rename getSortByTag to getSortByIndex, since it did not actually sort
by tag at all; it sorts by index of workspace tags in the user's config.
* Create a new getSortByTag function which actually does sort
lexicographically by tag.
* Enhance documentation.
algorithm
- idea is to specify sorting algorithm from user's xmonad.hs
- xinerama sorting algorithm produces same ordering as
pprWindowSetXinerama
- default ppSort is set to getSortByTag, so the default functionality
is the same as it was before
