Add custom field to media items in media library for associating tracks

[joedolson]

Can then add support in playlists to use captions etc in playlists.

[joedolson]

A few different custom fields to add:

1. Caption files for audio (multiple, with track types)
2. Poster images for audio.
3. Caption files for video (multiple, with track types)
4. Poster images for video.