TrackChosenByPlayer

The player changed which track of kind plays, on its own rather than because a caller asked; track is the one that plays now, or null for none.

It does this for a sound or subtitles that appeared after the open when none played or the one that played fell silent (#509), and for the subtitles that go with a new sound while the open's own subtitle choice stands (#506). A track a caller chose is never changed this way, except a sound the media stopped carrying.