Metadata getters and setters

The setters receive a pdfout_data argument.

The getters return a pdfout_data object.

E.g.

 void
 pdfout_page_labels_set (fz_context *ctx, pdf_document *doc, pdfout_data *labels);
 
 pdfout_data *
 pdfout_page_labels_get (fz_context *ctx, pdf_document *doc);

Remove the data if the setter is given a NULL argument. Return a zero-length pdfout_data object in the getter, if the data is not contained in the PDF.