* Re-enable MIDI export
* Fix logic for processing BB tracks and BB notes
* Consider master pitch and base note in MIDI export.
* Cut BB notes at the end of BB pattern.
This prevents a race condition with Qt5. A foreach loop makes a copy of its
Qt container, increasing the reference count to the container's internal
data. Qt5 often asserts isDetached(), which requires the reference count to
be <= 1. This assertion fails when the foreach loop increases the reference
count at exactly the wrong moment. Using a range-based for loop prevents an
unnecessary copy from being made and ensures this race condition isn't
triggered.
