Infinite playlists

[Sheraff]

Similar to a Spotify "radio" or a Youtube "mix", we could have "infinite playlists" that automatically remove played tracks and add new ones.

Would be nice for:

• full library shuffle
• liked tracks
• not-listened-to-in-a-while tracks
• new tracks
• ...

We could

• auto remove tracks that are N-3 before the current (to still allow for "back" button)
• auto add tracks when playlist is below a certain number
• keep a list of "already added tracks" so that the query to add more doesn't repeat tracks

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If we get this working, we could also implement a "find a mood" function:

shuffles through 10-second previews of seldom-played tracks in your library continuously, until you decide on what to listen to next