Enable software info again

Somehow the pdf.info = .. has no effect anymore.
Implement spot colors (basics)

This is currently working only for <Box> commands and must
be enhanced for all other occurrences of colors.
spot colors revisited

This is a cleaner implementation. Every spot color that is used
gets added to all following page resources, so we don’t need
to keep track which colors are used on a particular page.

While this might be a small waste of space, we make our lives
much easier with this implementation.
Colors can have overprint enabled

Overprint is now part of the color definition (pdfstring) and
switches to graphic state 0 (/GS0 gs).