Support playing subsongs

[runlow]

Expected You can play the songs contained within the songs.

Actual There doesn't seem to be a way to do that.

Steps to reproduce

1. Go to https://modarchive.org/index.php?request=view_player&query=38420
2. In the "Subsongs" dropdown select "Song 1", "Song 2" and "Song 3"
3. Go to https://coolmodfiles.com/?trackId=38420 (notice the same id)
4. Try to play the "subsongs" - there doesn't seem to be a way

Possible solutions Maybe if subsongs are detected then another "next" button would show up somewhere that would just switch to the to the next subsong. Maybe on the bottom, beside the playlist button or the loop button.