Add support for eventtitle, eventtitleaddon, eventdate, venue

[tobiasBora]

This PR https://github.com/plk/biblatex/pull/688/files (linked with this issue plk/biblatex#687) added support for eventtitle, eventtitleaddon, eventdate, venue in @unpublished. Unfortunately, these fields are not recognized in the trad-alpha style. Would it be possible to support them in trad-alpha as well please? (or if it's not possible for backward compatibility reasons, create a new style that mimics trad-alpha)

Thanks!

[moewew]

biblatex-trad is modelled after the base BibTeX styles which do not support these fields, so it is a bit tricky to add them. See also the philosophical discussion in #33.