Add option to add css class to fieldset legend element

Webform 8.x-5.x (for Drupal 8) is unsupported. A patch or merge request may be considered for the development branch by the maintainers if it is needed there.

Updated the version. Meant to choose the latest version.

I am not writing new features for the webform module. If someone contributes code with test coverage, it will most likely be committed.