Add plugin for tracking (non linked) images inserted with WYSIWYG Image

Problem/Motivation

The module tracks links but it will be useful to track images added with CKEditor Image plugin.

Proposed resolution

Add a 'CKEditor Image' plugin to be able to track images (files) added with CKEditor Image:

$xpath->query('//img[@data-entity-type and @data-entity-uuid]')

User interface changes

New 'CKEditor Image' option under the 'Enabled Tracking Plugins' on the 'Entity Usage Settings' config page.

Data model changes

New configuration key 'ckeditor_image' for 'track_enabled_plugins'.