Frame API cleanup, ffmpeg io enhancements.

[toots]

This PR does some cleanup around the new media/frame API:

• Frame should really only have one track mark per frame so it's better to had a track mark insertion API that removes all other track marks.
• Technically, frame can have more than one metadata but it's also better to avoid it so the API is changed accordingly

The ffmpeg IO input is cleaned up accordingly: only one track mark is set per frame and the same for metadata.

Lastly, icy metadata in ffmpeg input can be all over the place so we store them an add them back with each "normal" metadata. With deduplication enabled, this makes the metadata output much more clean.