Add support for audiobooks

[Mark Williams]

It would be very helpful to add a distinct library (rather than just a playlist) for audiobooks, for the following reasons:
1) No one likes their audiobooks to get shuffled in with their music when playing
2) Audiobooks often contain chapters, so the UI might need to change.
3) Syncing with an iPod requires that audiobooks go to a separate database on the iPod.
4) People often keep their audiobook collections in a separate library from their music and podcasts.

[Artem Anufrij (artem-anufrij)]

I have a big library of audiobooks. It would be useful to have a option for separating audiobooks and music.