Add to playlist mode

[NoahSaso]

Create a mode where you can select a playlist, and tapping on songs while this is enabled will add them to the playlist instead of queueing/playing them. Ideally this just lets users navigate around the interface normally, but changes the behavior of selecting songs.

It would also be nice to have the ability to preview songs or play them while this is enabled so someone can decide if they want to add the song without having to toggle the mode on/off.