Split internal and public functions for xmarks

To get xor sorted is a lot of work! To get back into it, a first step is
to look again at xmarks and update to current 'best practice' approaches.

I'll be raising some thoughts here on the team list before the end of the
year.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5481 de43f980-851b-0410-b2f7-c40aca1f87e0