Prompted playlist song replace

[MrSquid]

When working with prompted playlist you can have the whole playlist refreshed and get a new set of songs. Sometimes though the playlist is really good except that one song you want to swap. 

My idea is to allow swapping out a song based on the original prompt for the playlist without refreshing the whole list. 

Expanding that a step further may be to let users "lock in" songs on a promoted playlist so more than one some could be refreshed while keeping those "locked in" favourites.