let users customize colors for mistaken keys

[tomcucinotta]

this is for now hard-coded in tutor.cpp:

int def_colors[3][3] = {
  { 16, 0, 0},  // default color for mistakes
  { 16, 0, 16}, // default color for even channels
  { 0, 16, 16}  // default color for odd channels
};

but it should be customizable through the piano tutor panel, by using a RGB color chooser, and saving it along with the other save-able piano tutor options.