Jean Felder authored Sep 05, 2018 and Jean Felder committed May 29, 2019

Playlists can have the same song several times. Therefore, the song id
is not enough to have a unique identifer.

Use the song position as a suffix to trackid to handle duplicate
songs.
A current_song_index property is introduced to have the equivalent of
the current_song property.

Related: #100