Shuffle & Add To Playlist: reshuffle "tracks that have not yet played" except "play next" stack

[Sheraff]

Currently, when adding a track to the currently playing playlist with shuffle on, we re-shuffle the entire playlist (with current track set as the first).

A better / more "expected" behavior would be

• leave already played tracks as is in the order (up to and including cache.current shouldn't change)
• leave "string of uninterrupted tracks that are in the play next stack" as is in the order
• shuffle the rest

[Sheraff]

Not reproducing, needs more explanation.

Tried:

• start album, play
• activate shuffle
• forward twice, order of plays was 1, 8, 5
• "add next to playlist"
• backward twice, forward thrice, order was still 1 8 5 (and new one)