URL not sumbitted (missing context in JS behavior detach

I created 2 branches:
- one that simply adds the missing context in JS
- one that replaces the URL autosubmit and JS with a "Add URL" button.

I opened a MR for the 2nd option: replace the URL autosubmit and JS with a "Add URL" button.
Because this is a much better approach for UX: an autosubmit on input change is very intrusive for the user and a button is a much better approach.

I did have to refactor the #access applied on elements because having it to FALSE causes havoc.
It seems in that case Drupal doesn't understand what button triggered the action in \Drupal\Core\Form\FormBuilder::handleInputElement and considers the first button in form_state was clicked. This causes all kinds of misbehaviors.

This patch can be combined with 🐛 PHP warnings from FileUrlWidget Needs review which fixes another error with the widget.
Maybe it's best to centralize both in a single issue?

It works for me, but there is a question regarding the supported Drupal version.