Stop CKEditor IndentBlock plugin from being enabled within a text format when the IndentBlock plugin library hasn't been installed

While CKEditor IndentBlock module enabling shall be allowed while the IndentBlock plugin library is not available at any of the libraries paths, the IndentBlock plugin shall not be allowed to be enabled in any text format.

Problem

Currently, if the IndentBlock plugin cannot be found at any libraries path, the 'CKEditor plugin settings' tabs are incompletely displayed. E.g. the 'Image Uploads plugin' tab is not displayed, if the IndentBlock plugin is not found at a libraries path. E.g. the default path points to 'libraries/indentblock/plugin.js' as a last resort, but plugin.js is also not available at that default path.

Reason for this behaviour seems to be that a missing plugin makes the plugin detection end unexpectedly and not continue with other ckeditor plugins. It also doesn't help for returning an empty URL for the plugin path as this also breaks the plugin detection.

Solution

1. Disable the 'Indent Block' Enabled checkbox under 'CKEditor plugin settings' within any text format, if the plugin is not available at any libraries path, so that it can't be enabled and display a remark, that the IndentBlock library plugin cannot be found at any libraries path.
2. If the IndentBlock plugin library was removed, after it had been installed for one or more text formats, uncheck and deactivate the 'Enable Indent Block' checkbox programmatically like above, so that it can't be checked for re-enabling.