D14816: Add signature panel

[Chinmoy Ranjan Pradhan]

chinmoyr added a dependent revision: D13484: Show signature validation summary 
as popup.

REPOSITORY
  R223 Okular

REVISION DETAIL
  https://phabricator.kde.org/D14816

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