Refactor API and deprecate print_to_pdfa/2

[maltoe]

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)