macosx: Improve secondary text in library collection view items

In master we always display the duration of the relevant item, either in terms of seconds or in terms of numbers of items, as the secondary text in a collection view item. This is not always the best thing we could show, though -- e.g. for albums and songs it is more sensible to show the artist, for a movie the director, or for a TV show episode the name of the show.

This PR improves the detail strings of collection view items to adapt to the given media item subtype.

Signed-off-by: Claudio Cambra developer@claudiocambra.com