Bug Fix: NoteOn during final rest-sequence now stops performance

In an assisted performance, if the final subsequence is a restSequence
(the performer's key is up while the performance is finishing) and the
assistant receives a NoteOn event, the performance now stops.
Before this fix, Jazz crashed in this situation...
Bug Fix: The SVG 'Save' button no longer gets created twice.

This fix checks first, to see if it already exists, before creating a
new button.