Support duplex entry in ViewerPreferences

The attached SO question inspired me to support the duplex entry in ViewerPreferences, even if it isn't what the person asked for. Adobe Reader supports it, i.e. their print dialog changes depending on the entry.

To test this feature, create a PDF with 2 pages and use this code:

PDDocumentCatalog documentCatalog = document.getDocumentCatalog();
PDViewerPreferences viewerPreferences = new PDViewerPreferences(new COSDictionary());
viewerPreferences.setDuplex(PDViewerPreferences.DUPLEX.DuplexFlipShortEdge);
documentCatalog.setViewerPreferences(viewerPreferences);

This will show the calender duplex mode in the print dialog.