BackedNode.vala: cosmetic; wrap long conditional
Document.vala: document copy_node ()
Node.vala: improve documentation, note behaviour when working with nodes from other documents
Serializable.vala: add a serialize () and a deserialize () interface method, in case there are object-level serialization tasks that aren't contained to individual properties.
Serializable.vala: move serialize_property higher up in the source file, wrap its signature
Serializable.vala: wrap deserialize_property's signature