New branch for the refactoring of the print module, with the following objectives:
1. print_ui module to handle all the common link handling functions
2. make print.tpl.php a real template, moving all the HTML out of the module
3. hooks in print_pdf for pdf libraries sub-modules (print_pdf_tcpdf, print_pdf_dompdf and print_pdf_wkhtmltopdf)
4. add print_pdf_mpdf
5. add print_epub