Adapt to poppler sign api change

Now poppler offers signing via PDFConverter class. Add some error
handling, and store signature in temp file. Document gets reloaded
after replacing original file.
2 jobs for !36 with insert_digital_signatures in 15 minutes and 44 seconds (queued for 2 seconds)
latest detached
Status Job ID Name Coverage
  Build
passed #17652
build_ubuntu_18_04

00:10:05

passed #17651
linux kf5-qt5 qt5.12

00:15:44

41.0%