Unified document model

[winged]

We should introduce a unified document model, pulling together the "structure" and the "form jexl" perspectives. Currently, they depend on each other, but don't share enough data to be useful. See my comment in #1523:

I sort-of dislike that this introduces a coverage hole that wasn't there before, but on the other hand it's obvious why it's there. Also we can't really remove this code, as it is useful in certain cases (debugging, ...).

We should probably build a unified model of a form/document's structure that takes visibility jexl, calculated fields etc into account. Not today though.

Originally posted by @winged in https://github.com/projectcaluma/caluma/issues/1523#issuecomment-855998697