Remove the widget dependency on OptionsButtonsWidget

Problem/Motivation

Upstream changes in "OptionsButtonsWidget" recently destroyed our widget. We are currently doing a lot of workarounds in our widget class "formElement" method already, we should simply drop the upstream dependency and bake the parent code cleanly in our widget, before it is getting destroyed again after a major drupal update.

Steps to reproduce

Proposed resolution

Remove the widget dependency on OptionsButtonsWidget.

Remaining tasks

User interface changes

API changes

Data model changes