PDF forms export

We can export static versions of a document as PDF right now, but it would be nice to also export PDF forms, which can be filled in and maybe imported back in the wiki.

How this should work:

• Design forms in XWiki, like the current Sheets
• Export such a form in PDF
• Users download the PDF and fill it in
• They can import the PDF back in, where the information is extracted and transformed into an XObject.

Of course, not all types of fields can be filled in, as I don't think that DBLists can be exorted.

I don't know what tools we need to do this. Maybe it's a proprietary Adobe technology, in which case there's nothing we can do.