Create test that tests custom form with IEF element and make it pass.

Initialize #ief_labels for edit action

Split submit logic and move code to field widget and IEF handler

Validation functions cleanup

Decouple field widget settings from InlineEntityFormHandlerInterface classes

Refactor EntityInlineEntityFormHandler a bit and move some code from procedural to OO code

Don't extend EntityForm with handler and eliminate inline_entity_form_entity_form_validate() in favour of InlineEnttyFormHandlerInterface::entityFormValidate().

General clean-up

Form functions cleanup

Procedural cleanup

Remove redundant code from EntityInlineEntityFormController.

This reverts commit ad26b5b7.

This reverts commit b3c25a46.