Update for the two latest feature requests:
- The folder selection at each export seems to be non-trivial. The "Get Messages" style button is not available for add-ons. An alternative would be an option in the context menu. However, I couldn't figure out how to get a nested context menu. So I didn't implement it for now.
- Header information at top of the note: There is now an option to enable the info. The layout is hardcoded for now. I tried Nunjucks and really liked it. The implementation worked, but there is a warning about user defined templates. Not sure if it's a security risk in this case, but I didn't add it for this reason.
I uploaded the add-on in this state to the Thunderbird store. It's waiting for review.