Define content encoding rules

[solsson]

Need a reliable and unit tested solution for https://github.com/simonsoft/webui/issues/205. Must be safe to persist. This should be an API call available to uniteditors. Basically it takes a DOM node and produces a string (and possibly sets that string as a specific unit attribute).

[solsson]

Authormodel already has an XML lib for the serialization. Also it might be interesting to use something like https://github.com/solsson/innerXHTML.