Allow forms opened in AJAX dialogs to return results to the outer form

Problem/Motivation

When configuring an entity reference field, there may not yet be a bundle to reference, see #3033652: Be more helpful if there's no bundle to choose from.

The same can happen with comment types when adding a comment field, see #2987960: Comment fields are available to add to media, user or taxonomy entity types but this doesn't work.

For many site builders, adding their first fields is their first encounter with Drupal’s Field API. They should be able to create what is missing and carry on. Having to leave an unfinished form and to search the missing configuration somewhere, then coming back and starting afresh should not spoil their first experience.

Proposed resolution

We should let the user create the missing bundle, comment type, or whatever else they need in a dialog, then continue configuring the field with the newly created item. Deploying such a dialog should neither consume substantial time nor substantial technical infrastructure.

As the field configuration itself may already be in a dialog, its unsaved input must be preserved while the inner dialog is open, and the result returned to the outer form.

Remaining tasks

Technical review, Javascript review, accessibility review, manual testing.

User interface changes

None out of the box.