Add API for signature form field, and other helper classes

Summary:
This adds class FormFieldSignature which will represent a digital signature,
class SignatureInfo which will store signature validation information and
class CertificateInfo which will store details of the signing certificate.

Reviewers: aacid

Subscribers: okular-devel

Tags: Okular

Differential Revision: https://phabricator.kde.org/D14654