re #38, put color as const for key pressed
close #38 we now have the key pressed and released for keyboard
and the functions to know if it's a black or white key
and which keys correspond to the midi 'note number'
re #39 added a method in Context to link
a track id to a color
added utility functions in libmidi event
re #39, we now link the press/released keyboard event to midioutput