D14654: Add API for signature form field, and other helper classes

[Chinmoy Ranjan Pradhan]

chinmoyr added a dependent revision: D14655: Reimplement signature form field 
and related utility classes in poppler generator.

REPOSITORY
  R223 Okular

REVISION DETAIL
  https://phabricator.kde.org/D14654

To: chinmoyr, aacid
Cc: okular-devel, ngraham, aacid