[PP-1] Keep original field order of parsed PDF

Unlike FillPDF's LocalServer backend, pdftk returns PDF fields in the original order, page by page and following the tab order.

In 8.x-4.x we're keeping this order intact, as fields have a unique autoincrement ID rather than the composite primary key they have in 7.x-1.x. While we do #3049595: Consider composite primary key for FillPdfFormFields → , we would at least keep the numeric key as a weight column there.

To achieve the same, we'd need to introduce a weight column, too.

The main argument is that while sorting the fields alphabetically may be an option, the original field order is pretty helpful, particularly with many fields, to make sure adjacent, possibly logically grouped fields are not torn apart.

Please note that there's a separate ticket open for LocalServer: #3048514: Parsing a PDF form using LocalServer, fields appear in pseudo-random order →