WebSubmit: initial support for converting to PDF/X
- Offer basic conversion to PDF/X when possible, using Ghostscript. The "CLI" currently only converts to "pdf/x-3:2002", which is the version officially supported by Ghostscript. The Python functions pdf2pdfx(..)' and ps2pdfx(..)' can be used to switch to alternative profiles "pdf/x-1a:2001" and "pdf/x-1a:2003". Handle conversions in similar way to PDF/A.