The semitones distance of this Note relative to Note.c.
Example:
Note.c.semitones == 0 Note.d.semitones == 2 Note.f.sharp.semitones == 6 Note.b.sharp.semitones == 12 Note.c.flat.semitones == -1
@override int get semitones => baseNote.semitones + accidental.semitones;