Insert digital signatures
This is the 1st step in inserting digital signatures into PDF documents.
- adds 'Sign' menu entry and a toolbar button
- adds some configuration items of user identities to sign with
- adds a way to configure a NSS certificate database
- enables user to mark a place where signature field's going to be inserted
- applies the signature via poppler
You need the following poppler patches for this:
This MR supersedes https://invent.kde.org/kde/okular/merge_requests/36 - it appears I cannot amend that one.