Refactored the audio engine

I’ve streamlined the note processing logic in the audio engine. So far, the audio engine handled sequenced notes (notes that you can see in the editor) and the realtime MIDI notes that you can play using some MIDI controllers differently. It also lacked maintainability.

That’s why I’ve completely rebuilt the logic to unify the process. It also contributed to reduce the amount of code, and I’m happy with that.