implement midi server for version 2.
作業メモ
implement song data models, midi client and player.
skeleton of application
update .gitignore
rendering piano roll view
test environment
select{}.first => find
fix range bug
enable arrow keys
debug log
organize
move handling key input process to application class.
implement editor. just a little
log
undo/redo
input note number from keyboard
fix redo problem
implement tie & rest
octave shift