Add "Add to playlist" feature
This commit adds the possibility of adding songs to a playlist (as
discussed in #2). However, only the user's "own" songs can be added,
as can be seen in the SoundCloud API documentation ("You can only add
sounds that you own to a set"):

https://developers.soundcloud.com/docs/api/guide#uploading

Because of this, only "own" songs have an "add to playlist" button.