Fix bug in edit virtual field

The index was transformed twice from field index to field origin index,
resulting in a corrupted index (most often being 0 in the end).

src/app/qgsfieldsproperties.cpp

@@ -291,7 +291,7 @@ void QgsFieldsProperties::setRow( int row, int idx, const QgsField& field )
     QWidget* expressionWidget = new QWidget;
     expressionWidget->setLayout( new QHBoxLayout );
     QToolButton* editExpressionButton = new QToolButton;
-    editExpressionButton->setProperty( "Index", mLayer->fields().fieldOriginIndex( idx ) );
+    editExpressionButton->setProperty( "Index", idx );
     editExpressionButton->setIcon( QgsApplication::getThemeIcon( "/mIconExpression.svg" ) );
     connect( editExpressionButton, SIGNAL( clicked() ), this, SLOT( updateExpression() ) );
     expressionWidget->layout()->setContentsMargins( 0, 0, 0, 0 );