Some snippets have yet to be converted due to the more complex nature
of those snippets, and to keep things neat a separate commit makes more
sense for those.
While tackling issue #529, I stumbled across the odd behaviour that
re-compressing the archive with 7-Zip broke what should be a valid
Manuskript project.
After investigation it turned out that the code that loads the texts
sensibly expects there to only be files tracked in the files dictionary.
It is completely valid for a zip file to contain entries describing the
contained directories. The logical fix is to simply avoid adding these
directory entries to our files dictionary in the first place.
