Recalculate forms after command form changes

Summary:
notifyFormChanged is called by documentcommands when
Form fields are changed. E.g. by Undo / Redo.
We need to recalculate after such changes.

Test Plan:
Tested with simple_calculate example.

Run the unit test with the added line commented out and it failed, with the added line it passes.

Reviewers: #okular

Subscribers: aacid, ltoscano

Tags: #okular

Maniphest Tasks: T7805

Differential Revision: https://phabricator.kde.org/D10547