poppler: Convert Stamp annotations via C++

Instead of via the magic XML cycle of Poppler::AnnotationUtils::storeAnnotation + Okular::AnnotationUtils::createAnnotation
2 jobs for master in 23 minutes and 17 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #69584
clang_format

00:01:48

passed #69583
linux kf5-qt5 qt5.12

00:23:17

44.0%