Generalize DrupalImageCaption to work with elements other than image

Problem/Motivation

The filter module from core provides a filter to display image captions and align images: FilterCaption. This filter in itself does not requires the DOM element to be an <img> tag, but in tips, description, comments etc. it mentions the term image. Although, in short description it mentions that it can be used elements other than image:

return $this->t('You can caption (data-caption="Text") and align images (data-align="center"), but also video, blockquotes, and so on.');

But when this is used by the CKEditor module in 'drupalimagecaption' plugin (DrupalImageCaption and ckeditor/js/plugins/drupalimagecaption/plugin.js), it restricts its usage to just images. Although with few changes, this plugin can be generalized to all elements. By doing this, we are saving a lot of code from being replicated by modules which want to provide a similar caption/align features (for instance entity_embed, and maybe video also).

Proposed resolution

First task is to generalize the FilterCaption documentation so that it uses the term 'entity' instead of 'image' and modify/add examples to explain that div will work for elements other than image.

Next task will be to generalize the CKEditor plugin to remove its dependency on img element. Also, in case we do this, we'll want to rename 'drupalimagecaption' to 'drupalentitycaption' or something similar.

Since this filter is used in a couple of tests also, they will also need to be changed, but that should be as simple as changing the name of the class.

Remaining tasks

Write a patch.
Test it.

User interface changes

None.

API changes

None.