TrackContainerView: fixed dragged presets into BB Editor

When dragging preset files into BB Editor the patterns were messed up
because simple serializing was not enabled (and thus newly created
patterns were removed again). Fix this by enabling simple serializing
mode before calling InstrumentTrack::loadSettings().

Closes #3041686.

src/gui/track_container_view.cpp

@@ -409,6 +409,7 @@ void trackContainerView::dropEvent( QDropEvent * _de )
 		InstrumentTrack * it = dynamic_cast<InstrumentTrack *>(
 				track::create( track::InstrumentTrack,
 								m_tc ) );
+		it->setSimpleSerializing();
 		it->loadSettings( mmp.content().toElement() );
 		//it->toggledInstrumentTrackButton( true );
 		_de->accept();