Add possibility of switching modes without killing sound

[rblard]

In the web-app, it is now possible to shift modes freely at the press of a button. However, because this requires setting the index, and because setting the index executes the note events callback with an all-notes-off vector, this means that the transition is never quite fluid.

Potentially, this calls for the need to either :
A. Set the index without stopping, or
B. Stop without killing sound

Either of which could introduce bugs in other contexts.

[josephlarralde]

This issue plus some discussions we had before about your mode shift implementation seem to indicate that there is something clunky about this part of the library's API, and that it might need to be rethinked a bit (and while we're at it, this could also be an opportunity to add the MIDI track information to the event sets).