Extend key property variants

As these are not really easy to do using standard tools, we need a more-or-less
complete set. I've left `\generate_choices:n` as this is likely to be
deprecated. Note that for code arguments we have n/x versions only, while
for 'general stuff' I've gone for n/V/o/x, as f-type seems relatively
unlikely.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4511 de43f980-851b-0410-b2f7-c40aca1f87e0