GomElement: Implemented read unparsed children

If GomObject.parse_children is false, all children
are stored as string in GomObject.unparsed, without
parsing them, so no childs are added to current node.

GomObject.read_unparsed() will parse all children
stored in GomObject.unparsed, appending them to current
node as child. Once children have been parsed,
GomObject.unparsed is set to null to release resources.