Support additional widget actions in PDF Forms

Summary:
This adds support for actions associated with form fields
through corresponding annotation widgets.

Test Plan:
Still needs a unit test, only tested manually with
the document attached in the task.

Reviewers: Okular, aacid

Reviewed By: Okular, aacid

Subscribers: okular-devel, aacid

Tags: Okular

Maniphest Tasks: T8627

Differential Revision: https://phabricator.kde.org/D12665