QmlDesigner: Fix rubberband

It should reparented because at a model detach it will be removed from the
scene and deleted.

Task-number: QTCREATORBUG-12504
Change-Id: Id48c8d8f8a29b7aab60dca708bbefdac8c1f6024
Reviewed-by: Alessandro Portale <alessandro.portale@digia.com>

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

@@ -59,11 +59,13 @@ void SelectionRectangle::clear()
 }
 void SelectionRectangle::show()
 {
+    m_controlShape->setParentItem(m_layerItem.data());
     m_controlShape->show();
 }
 
 void SelectionRectangle::hide()
 {
+    m_controlShape->setParentItem(0);
     m_controlShape->hide();
 }