Metadata handling between different formats should be improved

[schierlm]

Different Bible formats provide different kind of metadata (like author, description, publisher, language, copyright, etc.).

At the moment, during import everything except the book title is added to a "Metadata book" at the beginning of the exported Bible. During export, this "Metadata book" generally gets added as prolog before the actual bible books, or gets written into a single metadata field of the destination format.

This way, no metadata gets lost, but when converting e. g. Zefania XML to OSIS (as pointed out in issue #8), there is often manual work required to clean up the metadata of the modules after conversion.

This leaves some way to improvements.

[Contributions / patches for this issue greatly appreciated]