add update accidentals policy

Change accidental while pressing alt key makes new acccidental valid for the rest of measure or until another accidental was found (as I need in most cases).

Sorry, I just see that I pushed some garbadge (some unnessessary lines, code from another idea that is commented out, a whole .mss file used for some tests,... Shoud we cancel this pull request, and Iclean up and push again?