Editable widget in readonly mode is editable

[Radoslav Petkov]

Steps to reproduce

1. Open widget/dev/nestedwidgets
2. Add the following code in the body:

<button onclick="setReadOnly()">SetReadOnly</button>

Add the following in the script tags:

function setReadOnly() {
		var editor1 = CKEDITOR.instances.editor1;
		editor1.setReadOnly(!editor1.readOnly);
	}

Expected result == Editor1 (the iframe sample) should be in readOnly mode and the editable widget should not be editable

Actual result == The widget is editable

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

[Jakub Ś]

DUP of #12134