Move some prefs under the new-ish Audio Prefs screen. h/t @ljunkie

It's kind of a tossup, since the prefs *are* advanced. But now that we
have an audio prefs screen anyway, it makes sense for these to live
there.
Add a toggle for DTS support, fixes #267.
Only use the universal transcoder with items that have streams.
Make sure we have video details before trying to play, fixes #265.