Memory: Node.vala, BackedNode.vala, Element.vala, NodeList.vala, Attr.vala,...

Memory: Node.vala, BackedNode.vala, Element.vala, NodeList.vala, Attr.vala, Entity.vala, Document.vala: change ownership of return value to unowned for a bunch of methods (insert_before, append/remove/replace_child); references will be handled by the Document