Refactor API and deprecate print_to_pdfa/2

Now with the "multiple sources" feature being in place, it becomes apparent that the `print_to_pdf/2` / `print_to_pdfa/2` separation wasn't a good call. Refactor as follows:

* Make PDF/A conversion an optional step of `print_to_pdf/2`, dependent on presence of new `pdfa: true` flag
* Deprecate `print_to_pdfa/2` (route it to `print_to_pdf/2` with `pdfa: true`)