Change internal structure of props.

This commit adds one token to the start and one token to the
end of every property list.  Additionally, it goes towards an
approach closer to l3seq's \__seq_item:n.  The next commit
should thus bring some speed-ups for inline property mappings.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4562 de43f980-851b-0410-b2f7-c40aca1f87e0