Prevent enabling a text editor if a text format has one or more FILTER_TYPE_MARKUP_LANGUAGE filters enabled

Problem/Motivation

Nonsensical situations that are currently not prevented by the UI:

1. Enabling CKEditor for a Markdown-powered text format.
2. Enabling CKEditor while the filter_autop (or filter_url) filter is enabled.

(If it doesn't make sense to you that using filter_autop and filter_url precludes you from using CKEditor, then please read #1911884: Enable CKEditor in the Standard install profile → and 📌 Prevent enabling a text editor if a text format has one or more FILTER_TYPE_MARKUP_LANGUAGE filters enabled Closed: works as designed FIRST.)

Proposed resolution

As of #1782838: WYSIWYG in core: round one — filter types → , filters can have types. filter_autop and filter_url are marked as FILTER_TYPE_MARKUP_LANGUAGE, because they are in effect extremely minimal markup languages.

Whenever >=1 FILTER_TYPE_MARKUP_LANGUAGE-type filter is enabled, we should disallow the enabling of a text editor.

Remaining tasks

1. Get feedback. Figure out how to deal with edge cases, like http://oscargodson.github.com/EpicEditor/ — a Markdown "WYSIWYG" editor.
2. Write the code. Should probably use Drupal core's dialog and/or #states.

User interface changes

TBD

API changes

None.