fix: reimplemented SustainKey in a much more cleaner way (with a ModifierKey abstract class) + added CC (alternative to Note for modifier keys) + added mSpecialColor (for modifier keys) in all themes + fixed bug that prevented Sustain label to be displayed + fixed bug that prevented the stopping of notes pressed twice when disabling sustain + added label type Key Number (DEV)

Signed-off-by: Stephen L. <lrq3000@gmail.com>