Improved bannertopdf() filter function

Improved bannertopdf() filter function

Main reason of the improvements is to make the bannertopdf() filter function
more suitable for Printer Applications, conserving its functionality for
CUPS.

Changes:

- Instead of a pair of an instruction file
  (application/vnd.cups-pdf-banner) and a template PDF file we can
  only use the template PDF file now, sending it as input file to the
  filter function instead of the instruction file. The instruction
  file content can be added to the PDF file as PDF comments,
  preferrably at the end. If no instructions are added to the template
  file, default instructions are used.

- Job-/Printer-related text added to a template file without PDF form
  (according to "Show" instruction) is now cleaned up, not showing lines
  at all for fields for which no content is supplied, instead of showing
  "(null)" or nothing as content.

- Page dimensions are now also shown in centimeters and not only in
  inches.