WIP: [cmake] Switch to UseLatexMk

I had to break backwards-compatibility with regard to add_latex_document because there is a name clash. Both UseLATEX.cmake and UseLatexMk.cmake define this function. I renamed the old one, because the new one becomes the new interface. dune_add_latex_document can be removed after 2.7.

I tried this with dune-common, dune-grid-howto, dune-istl, and DuMuX. Works fine. Only problem occur with empty indices. I had to drop the according commands from the LaTeX documents.

Check !594 (merged) which seems to be the preferred way.