Negative Z will stack an item behind its parent in the formeditor

Task-number: BAUHAUS-618
Reviewed-by: kkoehne

src/plugins/qmldesigner/components/formeditor/formeditoritem.cpp

@@ -83,6 +83,7 @@ void FormEditorItem::setup()
     setContentVisible(qmlItemNode().instanceValue("visible").toBool());
 
     setFlag(QGraphicsItem::ItemIsMovable, true);
+    setFlag(QGraphicsItem::ItemNegativeZStacksBehindParent, true);
     updateGeometry();
     updateVisibilty();
 }