Adding features to learn songs easier

[HelixPiano]

Is your feature request related to a problem? Please describe.
I'd like to see two features being added to Embers that exist in synthesia :

1. Slow down the midi file (e.g. to 50%, 60% etc.) during replay.
2. The option of letting embers only play the left or right hand of a song (given that the midi provides a separation).

Describe the solution you'd like
Adding these features would enable people to learn songs with the help of Embers easier. Slowing down the speed of the song or only practicing one hand is really useful.