Move some Media meta to the File class
Right now we're storing the audio-delay & spu-delay as a media meta, so it means we're losing the ability to store it per-track. We could move it as a File meta, and possibly also as a VideoTrack/AudioTrack/SubtitleTrack meta, so that we can store this kind of information based on the track associated to the media, not only once per media This could also allow us to know which attached input was selected during last playback.