"Toggle MIDI Output" needed, too (or even per-score?)

When a MIDI Output device is present, scores output to it unconditionally. That is, after you're done fooling around with your MIDI-specific score, and you download something completely unrelated and attempt to play it ---surprise! Whether MIDI output enablement should be per-score is another good question, but a control like the now-broken MIDI output control in the toolbar would be a first step.