Selectors passed to CKEDITOR.dom.element.prototype.find are not properly escaped

[Tomasz Jakut]

Steps to reproduce

1. Open ​https://jsfiddle.net/5487w3gc/3/
2. Click the button located under the editor.

Expected result

The alert with Test is shown.

Actual result

The alert with Error is thrown is shown.

Other details (browser, OS, CKEditor version, installed plugins)

The problem is located inside the getContextualizedSelector function inside core/dom/element.js file. It escapes only the id of the element. The proposed fix could split the selector on all special chars likely to appear in a CSS selector and escapes them separately: ​https://gist.github.com/Comandeer/de6c832993a8b9c21b01

Blocked on #14451 (as it introduces CKEDITOR.tools.escapeCss method).