notes should not be muted on keyrelease in editor

Played notes are muted when the key is released. This is usually ok, but in the pattern editor it would be useful to keep the note playing until another note or a rest is added on the same channel. This is useful as it teaches the user how the patterns work. The playing sounds should also be muted when stop button is pressed, so the user can kill the audio if she wished to stop editing. It may also be desirable to kill the audio when the user moves the focus away from the pattern editor.