This is the same concept as the fillpdf_file_context table in D7. It
lets the module
store generation info for generated PDFs so that private storage access
checking
can work properly. I'll be simplifying this a lot in D8 and simply
storing a
URL that I can parse to get the entities whose access needs to be
checked. And
I might backport that to D7; it's much simpler.

There's lots of changes here. I should have committed more often.
Anyway, the gist of it is:
  - General code conversion to Drupal 8 patterns. I've taken shortcuts here and there but mostly avoid that. No idea on the quality. Hopefully, someone can give me some feedback.
  - Two new entity types, FillPdfForm and FillPdfFormField. These
    correspond to the old {fillpdf_forms} and {fillpdf_fields}
    tables. Now they're finally entities, so Views support will
    be possible for those of you who wanted it. I was finally forced to
    do this, and I'm glad.
  - PDF-filling methods are now PLUGINS!!! This means they all
    implement FillPdfBackendPluginInterface, and this basically
    achieves the same thing I was trying to do with the failed
    pdf_forms project. Anyone who wants to add their own
    backend can already do it; the fillpdf_service_backend
    configuration variable is used to select the correct backend.
    It should match the annotation ID. Look at
    FillPdfServiceFillPdfBackend to get an idea of how to structure the
    plugin.
  - Better use of actual entity references. FillPdfForms store a
    reference to their underlying Files instead of just an ID. Same with
    FillPdfFormField -> FillPdfForm.