PlaylistEditor should play a list of songs

[theGreatWhiteShark]

I lost count. It might be the third time I thought I broke Playlist handling since Hydrogen does not jump to the next song after the previous one did end. But it turns out we never supported this.

As I rewrote large parts of the AudioEngine I kinda understand why this was not implemented in the first place. But I think we really should support this. Most people probably expect Hydrogen to play through all songs arranged in a playlist sequentially.

I know we have things like Pattern Mode and Loop Mode to take into account. Nevertheless, they do not feel like a problem. We'll probably find a nifty solution, like parsing all song files and displaying warning icons featuring P and the loop icon.