This adds a new track property in JSON/verbose identification mode
called "multiplexed_tracks". It's an array of track IDs that describe
which of the tracks mkvmerge reports as separate ones were originally
part of the same source track (e.g. TrueHD+AC-3 in a single track in
MPEG transport streams).
Implements #1835.
"playlist_file" must be an array of strings, not a single string. In
verbose identification output mode this property has always been output
multiple times. In JSON only one of those was actually output, the last
one. Additionally the JSON schema didn't contain "playlist_file" at all.
