InstrumentTrack: Fix issue #780 by showing the m_pitchRangeSpinBox too, in case it's been hidden.

src/tracks/InstrumentTrack.cpp

@@ -1323,6 +1323,7 @@ void InstrumentTrackWindow::modelChanged()
 		m_pitchKnob->setModel( &m_track->m_pitchModel );
 		m_pitchRangeSpinBox->setModel( &m_track->m_pitchRangeModel );
 		m_pitchKnob->show();
+		m_pitchRangeSpinBox->show();
 	}
 	else
 	{