Allow adding arbitrary XML to XML file

Problem/Motivation

See: #2886357-66: Changing default "" and "" tags in XML export → for more details.

XML Header (the metadata) This seems like a bit of a weird one to me, but I understand that there might be some use-cases out there that would want to add some arbitrary XML to the root node of the file. Again this should get added via the XML encoder if possible, but maybe having big warning messages all over this entry field that you're likely to break things unless you know exactly what you're doing, then maybe it's okay.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes