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