Fix stuck notes on panic or all-notes-off.

This adds a fix fundamental (Mark McCurry) and I came up with for stuck notes that occur when one
ZynAddSubFX gets a panic or all-notes-off when in mono/legato mode and notes are stacked up.
(I.e. multiple note-ons are received without note-offs)  I encountered this working with QTractor and
ZASFX, but the same problem should be present with the LMMS plugin as well.