Add reference and null node support to XML exports / imports
Created by: RayKoopa
The new YamlConv-like XML format requires support for referencing other nodes, to allow reusing them (aka "cached nodes").
Since the original YamlConv does not support this, find a good way to do so. Probably an encoded XPath or similar, already standardized by XML conventions. This also requires finding out if only complex nodes like arrays or dictionaries can be null, or primitive value node types too, to decide for an XML format for null nodes.