Only sound selected notes momentarily (#6142).

Only sound selected notes momentarily (#6142).

This is debatable, the "sustained until mouse release" behaviour is handy
sometimes, but this way seems like what most people probably want.

Also, this "fire it and forget it and let it delete itself a bit later" thing
with MidiPlayer makes me nervous.  I guess it's unlikely someone manages to
select a note then delete a track within 100ms, but, well...