Used to add content in an GXml.Element.
By default no contents is serialized/deseralized. Implementors must implement Serializable.serialize_use_xml_node_value function returning true in order to use this property.
This property is ignored by default. Implementors must implement serialize_use_xml_node_value to return true and add a set and get function to get/set this value, in order to use your own API.
This property is ignored on serialisation.