Add support for field hierarchies

[JorjMcKie]

This fix adds Form Field hierarchy support to our Widget handling.
Background:

PyMuPDF (like MuPDF itself) always inserts new Form Fields as widgets on the respective page. All inserted widgets automatically become root fields in the PDF.
There is no way to add complex information structures like Person.Name.First such that the widget only has the name part First and the other segments are parents in multiple hierarchy levels.

This fix solves this by automatically adding the hierarchy where needed. For the example above, a chain Person → Name → First will be created. Person becomes a root field pointing to Name via its /Kids array, and similarly Name points to First. Name and First in turn will point backwards to their parents via the /Parent key. This allows for tree-like information structures like this:

Person
  ↪ Name
     ↪ First
     ↪ Last
  ↪ Address
     ↪ Street
     ↪ City

The only root field would be Person. The four level 3 items would represent widgets living on any pages.
This behavior is triggered by the presence of the name segmentation indicator colon ".".
If no colon is present, the old behavior occurs.

[julian-smith-artifex-com]

[mupdf master and 1.28.x have just been updated with the fix for swig-4.5. We need to rerun test_quick after these changes have made it to the github mupdf mirror, so rerun in an hour or so.]