#1031: fixed inheritance of Author tags for properties

Due to a logic error the inheritance for author tags in Properties no
longer worked.
Moved inheritance of the common Author tag to the Abstract class.
Fix inheritance of @copyright and @version to be compatible with all elements
Add tests to check whether @version, @copyright and @author are inherited
Moved testing of the tag inheritance for author, version and copyright

Whether these are correctly inherited is the responsibility of the
individual elements. Testing the abstract class consolidates the
effort but negates the effect of actually testing the descriptor itself.