Set value to be set to a GXml.Attribute, to be added to a GXml.Element
If value is set to @null then the property will be ignored by default and no property will be set to given GXml.Element.
Some implementations stores the value without any convertion at all; then if the value, from XML property, makes no sense for the property type, you should take care to use the provided API from them to convert it.