API: enhance XML output

[peterstadler]

especially arrays are output without a wrapper element

[peterstadler]

implies several steps:

• add information about XML element names, namespaces, wrapping, etc. to the OpenAPI specification, see https://swagger.io/specification/#xml-object
• add support in local:serialize-xml() in the controller
• check whether other software (e.g. ant calls) are affected by the change of the XML format
• finally add official XML support to the response objects of the OpenAPI specification, see https://swagger.io/specification/#response-object