don't implode voices of different rhythms

as this it can lead to data loss.
On imploding staves it could lead to some incorrectness, but as the old
staves are still there, this can get spotted and fixed.
Fix 40901: Midi remote control Augmentation dot doesn't work
Merge pull request #1516 from rtega/404-new-feature

Fix 40901: Midi remote control Augmentation dot doesn't work
Merge pull request #1507 from Jojo-Schmitz/implode

don't implode chords of unequal duration