Rework Playlist API

The playlists were initially designed to hold unique media, which isn't the case anymore. This means we can't remove a single occurrence of a media that was inserted multiple times.

Another issue is that the position is in the [1;N] range, instead of the usually expected [0;N-1]

• Use a [0;N-1] position/index to add items to the playlist
• Curate the position records after deletion
• Change the API to delete media based on the position
• Change the API to move media based on the position
• Index the PlaylistMediaRelation table by playlist_id & position
• Migrate previous playlist to contiguous indexes
• This must be part of model 16, since model 15 is already used as part of vlc-android beta

This needs to be backported to 0.4.x afterward