First implementation for key filtering/selection by group

No docs just yet at least in part as I'm not 100% sure about the edge case
questions. For example, what is sensible w.r.t. unknown keys if filtering.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4555 de43f980-851b-0410-b2f7-c40aca1f87e0