Fix author parsing in Wikidata: Again! (the same, but different, but still the same)

[larsgw]

Parsing an entire set of entity data is fine, parsing a single prop (author by string or by Wikidata ID, P2093 and P50 respectively) is not. Also, the code to merge both author types is about half of the code of the method. Should be able to be done easier.

This is both a code-enhancement (because the general code looks ugly) and a bug, the latter because the method (not the code) to parse a prop is now exposed to the public and should be able to be used independently of the method to parse a full entity.

[larsgw]

Not a duplicate of #19, basically an improvement to the lazy fix for #21.