Include aritsts from Audiobooks & Podcasts in auto-fill

[psychoadept]

I notice that none of the artists I enter for Audiobooks are included in the auto-fill options.  Since some of those include radio shows with large numbers of guest artists, many of which reoccur, I would find it helpful for them to be included.

(Actually, I think it would be cool if the auto-fill for those categories was restricted to artists used in those categories, since there's not much overlap, but I imagine it's easier to just include all of them.)