improve referer mechanism

[JoernT]

currently the referer header is used to fetch the original HTML doc which hits the XFormsFilter as a form submit.

This is a flaky mechanism as not all servers provide the referer header or this gets even filtered out by some proxies.

One potential way of doing it (although a bit silly from the authors perspective) would be to add a special hidden input to the form which contains the file name and relative location. When using javascript setting this field would be trivial.