Used to check GXml.Element's contents must be deseralized.
By default GXml's implementations doesn't deseriaze/serialize XML node contents. In order to enable it, you must override serialize_use_xml_node_value method to return true and store XML node's content to serialized_xml_node_value property.
Implementors could set up methods to provide a clean easy to use API to set nodes contents. In most cases, users would like to set a value through a getter or setter or through a property in the class.