[WIP] re #21, now after we selected a song, it will play one track of the midi file in background,
try with 'brief fur elise' for a better result, lot of hack to fix
re #21, remove the '-2' hack, it was not needed at all
now when we switch from one screen to an other we clean the midi output so
we dont have pending note that continue to play
Merge pull request #22 from allan-simon/implement_21_play_midi_background

close #21 play midi background