Improve REST services config tool

The following improvement have been considered:

• Add support for exposing complex child nodes / references if they are XML-annotated as well.
• Provide a way to XML-annotate new fields (after adding a new document type field in the CMS, you can run bean re-generation, but this will not, currently, XML-annotate your new getter, even if other getters in the bean are annotated.
• Support the retrieval of individual documents, rather than (queried) lists of documents.
• Remove the UI selection of the REST endpoint type (we currently only support one type), or make the selection persist (if supporting multiple types).