Access readOnly state of FormWidgets dynamically

Summary:
This is more of a cleanup patch that removes the obsolete m_canBeEnabled
member variable which was a leftover IMO from a time where readOnly fields were
shown as disabled. readOnly fields are invisible, not disabled, and the code no longer assumes that
readOnly does not change over time.

Test Plan: Tested manually and with a unittest which is part of the series.

Reviewers: #okular

Subscribers: aacid

Tags: #okular

Maniphest Tasks: T8097

Differential Revision: https://phabricator.kde.org/D10865