Make ampache API action playlist_songs return the tracks in the correct list order
Make ampache API action playlist_songs return the tracks in the correct list order

- Previously, the tracks were returned in pseudo-random order and any duplicates were discarded
- Also, the track number of each track of the result set is now the track's ordinal on the playlist (instead of the value of the "track number" tag). Caveat: this doesn't work correctly if there are duplicate tracks on the list.

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