FYI Watermarking reworked

The watermarking script didn't work for me (didn't apply a watermark), so I radically simplified it to fit our needs. Imagemagick operates now directly on the pdf file. I think this speeds up the watermarking a lot, but I haven't measured it.

Based on

This removes a lot of the functionality, for example the A3 splitting. So I guess this MR is more for your information than intended to be merged