I'm getting an error when I actually try to populate/download the PDF. There is probably
something wrong with the Request object I'm returning. But this is closer.

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.

Conflicts:
	fillpdf.admin.inc

Way too much to list here. See http://drupal.org/node/1266174.

This patch also shows additional information on the Fill PDF overview
  page, namely the location of each PDF. This is to distinguish them
  since this patch also fixes the issue where identical filenames
  are not renamed.