edl: make it possible to delay-load files with multiple tracks

Until now, delay-loading was for files with single tracks only
(basically what DASH and HLS like to expose, so adaptive streaming and
codec selection becomes easier - for sites, not for us). But they also
provide some interleaved versions, probably for compatibility. Until
now, we were forced to eagerly load it (making startup slightly slower).

But there is not much missing. We just need a way to provide multiple
metadata entries, and use them to represent each track.

A side effect is now that the "track_meta" header can be used for normal
EDL files too.

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