Feature Request: Improved no LED learning - Highlight wrong (existing feature) and show correct note(s) (new feature)

[NoWayOut76]

My learning method is:

• Practice with lights on
• Turn off lights for one hand
• Turn off lights for both hands
  Currently this works fine, if i press a wrong note, it will light up red, even i selected "Black" color for the hand(s).
  But when i press the wrong one, i do not know which one i should have pressed instead

Suggestion for feaures:

• Toggle switch to enable / disable light for each hand (now i need to select black and then move back to original color which is a bit cumbersome)
• When "black" is selected and i press the wrong key, highlight the correct key(s) i missed to press in green in addition to showing the current wrong key(s) as red

[NoWayOut76]

I have added those features myself (trying to not mess up the code ;)) and created a pull request for it