Fix CheckBox script usage

Summary:
This implements setting / getting the value of buttons,
which is important for checkboxes in scripts. It also moves
the checkbox activate action after the value is set so that
the correct value is used when the activation script is
executed.

Reviewers: Okular

Tags: Okular

Maniphest Tasks: T8097

Differential Revision: https://phabricator.kde.org/D10868