Improve the UX of captioning in CKEditor

Problem/Motivation

In reviewing #2994702: Allow editors to alter embed-specific metadata, as well as `data-align` and `data-caption` → , the UX team had this feedback:

...the UX of the "Caption" checkbox is strange. However, it is a pre-existing pattern in core; the image button does it too, exactly the same way. We all felt it would be best for them to behave consistently. So let's open a follow-up issue to improve the UX of the captioning in the embed and image dialogs in the same go -- ideally, checking the "Caption" box would expose a text field with the caption in it, and it would be live-editable in the editor.

We need to figure out what we want to do here, then fix it everywhere.

Proposed resolution

Determine what a better UX would be, then implement that in the drupalimage and drupalmedia CKEditor plugins.

Remaining tasks

The next step is to implement the proposed solution: checking the "Caption" box would expose a text field with the caption in it, and it would be live-editable in the editor. In other words, allow users to set the caption in either the dialog or the editor, and ensure that both are synchronized.

User interface changes

Yes.

API changes

Doubtful.

Data model changes

Unlikely.

Release notes snippet

TBD