Allow setting *raw* chapter/section/subsection labels

[fingolfin]

I am converting parts of an existing GAPDoc manually to AutoDoc. For this it is important to keep labels the same, so that I don't have to change more than one file at a time.

But I couldn't figure out a way to set the label for e.g. a section to an arbitrary value. It seems AutoDoc insists on prefixing the section label with a string. So even if I do this:

@SectionLabel Foobar

I end up with this in the resulting XML:

<Subsection Label="Subsection_Foobar">

IMHO we should just drop these prefixes. Sure, that risks a clash in labels -- but so what, the same holds for LaTeX labels, for hyperlinks, kind of for anything in programming that requires names.