NodeList.vala: documentation fixes; link to GList when a method is based off of that, add some missing param fields

test/Serializa{tion,ble}Test.vala: adapt to new behaviour of gxml_serialization_{de,}serialize_object

Serialization.vala: API BREAK: change serialize_object and deserialize_object to work with actual GXmlDocuments, not just their root nodes.  This is necessary because a GXmlNode is not valid without a reference to its GXmlDocument.

DocumentTest.vala: invalid characters no longer cause create_element to return null, so don't test for it

Document.vala: don't return null on create_element if invalid characters have been specified; print a g_warning and proceed instead (libxml2 accepts invalid characters anyway)

CharacterData.vala: change behaviour when offset == length of string for bounds checking, since the end of a string is a fit place to insert things (but perhaps not remove :D)

Memory: Document.vala: node creation methods (e.g. create_element) now return unowned/weak references to the nodes they create; the Document alone owns them

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