Add refresh widgets if underlying field changes

Summary:
If a field is updated because of a calculate form action /
a script execution, not only refresh the rendered pixmap but
also the corresponding formWidget.

Test Plan: Unittest in separate revision. Tested it manually, too.

Reviewers: #okular

Subscribers: aacid

Tags: #okular

Maniphest Tasks: T7805

Differential Revision: https://phabricator.kde.org/D10048