Added signing of PDF documents with digital signatures

via Qt5 interface and pdfsig with parameter -s.

Includes fix to write document then update byte offsets and sig on disk
by Adrian Johnson

Includes compile fixes by Oliver Sander and Albert Astals Cid

Fixes bug #99416
6 jobs for sign-pdf-with-digital-signature in 4 minutes and 34 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #189321
build

00:04:11

passed #189326
build_android

00:01:01

passed #189322
build_clang_libcpp

00:04:16

passed #189325
build_clazy

00:04:34

passed #189324
build_mingw64_fedora

00:03:33

passed #189323
build_ubuntu_16_04

00:02:38