Analyze/improve how other modules can provide CKEditor plug-ins

quicksketch in #1874332-3: Convert to use the plugin system:

What do you think about hook_ckeditor_plugins()? Is that something that could (or should) be converted to D8's plugin system also? It's a bit different because if each CKEditor plugin were required to be a Drupal plugin class, we'd end up with ~20 classes that would all be empty. At first I thought because most *_info() hooks are going away in D8 (and hook_ckeditor_plugins() is still essentially an *_info() hook, even though it doesn't have "info" in its name), that using the plugin system would make sense. However on further thought, I'm not sure it's a good fit. Perhaps if each plugin provided a settings callback too, the plugin system would make sense. Right now the module is simply assuming the plugin will form_alter() the CKEditor configuration form to expose its options.