Allow adding to the playlist immediately after the current song
Reason for the change
Add the ability to add to the playlist immediately after the current track.
Notes
- I made it so that clicking this button with shuffle mode enabled will add the tracks unshuffled. I suppose some people might want to enqueue an album after the current track with the album internally shuffled, but I haven't implemented this. It might be possible, but I'm not sure how feasible it is with the current playlist design.
- Probably need to design a more specific icon.
- Needs lots more manual testing
Test plan
Added some autotests.
Manual tests:
- Add some tracks to the playlist
- Add some albums, genres, tracks, etc. using the new "Play next" button
- These tracks should always appear after the current track (even for shuffle mode)