Implement digital signature support for PDF
This is a squash of the original branch
It also contains fixes/rework from Albert Astals Cid
If you're interested in its history have a look at the
gsoc2018_digitalsignature branch
The poppler dependency situation is:
- With 0.51 you get most signature information
- With 0.68 you also get signature location/reason information
- With 0.73 you also get signature certificate information