Song speedups

[TheGHNerd2]

I think speedups would be pretty good for practicing skill with the easier songs, as some songs are really good for building up right hand finger independence but normally are too slow to where doesn't really help improve it but speeding those songs up would.

[NarrikSynthfox]

This may be possible, but it would require implementation of a time stretching library and loading the whole audio file for each stem into memory instead of streaming from disk