CKEditor custom plugin to add richcombo select list
You should really be extending CKEditorPluginBase and implement getButtons method. This is likely why you don't see it in the administrative interface, but in a vanilla CKEditor test, you do.

https://api.drupal.org/api/drupal/core!modules!ckeditor!src!CKEditorPluginButtonsInterface.php/function/CKEditorPluginButtonsInterface%3A%3AgetButtons/8.2.x