Check `required`-attribute for required (single) selection fields

[mutec]

select-fields support the required-attribute unter some circumstances in HTML5 documents: https://stackoverflow.com/questions/6048710/can-i-apply-the-required-attribute-to-select-fields-in-html5/6048891#6048891 https://www.w3schools.com/tags/att_select_required.asp https://html.spec.whatwg.org/multipage/form-elements.html#attr-select-required

see a8490749c3ba7014380d55462fc45dd635c1d71c This issue is supposed to check the whole project(s) in order to locate where the required-attribute can/should be set.

• [ ] __singleSelectionFormField.tpl
• [ ] acp/__singleSelectionFormField.tpl

to be continued…