use ODT 'Author' style to specify author

[raffazizzi]

This issue is derived from #327, which couldn't easily be merged. A new implementation is needed.

It should be possible to specify the Author of the document also by using the 'Author' ODT Style (alternatively to the hard-to-access dc:initial-creator meta tag).

The 'Title' ODT Style is already used to allow the specification of the title of the document (alternatively to dc:title). A change is needed to extend the same logic for the Author.

(in libre office meta fields are either hard to access or downright missing in the GUI).