Do not leak toolbox items

Finally found the cause of the items QML_LEAK_CHECK warns of.

BUG:336561
REVIEW: 119343

src/scriptengines/qml/plasmoid/containmentinterface.cpp

@@ -123,6 +123,7 @@ void ContainmentInterface::init()
             QObject *toolBoxObject = qmlObject()->createObjectFromSource(QUrl::fromLocalFile(pkg.filePath("mainscript")), 0, toolboxProperties);
             if (toolBoxObject && containmentGraphicObject) {
                 containmentGraphicObject->setProperty("toolBox", QVariant::fromValue(toolBoxObject));
+                toolBoxObject->setParent(containmentGraphicObject);
             }
         } else {
             qWarning() << "Could not load toolbox package." << pkg.path();