Add "title", "album" and "artist" to Audio result struct

[julik]

For previews it would be very useful to add shortcuts for displaying MP3/MP4 track titles - without having the caller pick them out of the intrinsics attrubute. There is also the unspoken convention that intrinsics can change between minor versions of FormatParser, which can be harmful for long-term storage of metadata. The top-level attributes for audio would be stable, meaning that they would be set regardless.