[ADD] XML export

Allow users to export data in a re-importable XML file

- Task: https://www.odoo.com/web/#id=8803&view_type=form&model=project.task&action=333&active_id=131
- Pad: https://pad.odoo.com/p/r.GQ9qOYpLqUjA9L7D
I'm talking about the *encoding*, not the content-type in general. Having a content type is good, I'm asking why the encoding is in both the content-type and the document itself.