Make parent object optionally explicit in Qt.createComponent()

When Qt.createComponent(...) is invoked in QML, the created component is automatically parented to the running QML engine. This prevents the component object from being collected even if there are no javascript references to the object. To allow the object to be cleaned up when no longer in use, the automatic parenting will be removed. It is therefore necessary to allow the creator of the component to optionally specify a parent object directly, to prevent unwanted collection.