Widgets gets deleted when editor is readonly

[Duskfall]

Steps to reproduce

1. Make editor readOnly in config
2. Use a widget like placeholder
3. Try to delete placeholder

Expected result

Do nothing

Actual result

Widget gets deleted in read only editor

Tested in 4.5.7 and 4.5.8

[Jakub Ś]

Problem can be reproduced from CKEditor 4.3 and is a part of a larger ticket #12134.

To reproduce:

1. Open placeholder sample
2. Make editor read-only with CKEDITOR.instances.editor1.setReadOnly(true);
3. Click on editor and press Ctrl+X or Delete
4. If that doesn't work - unfocus and focus the placeholder; next repeat step 3.

NOTE: In IE error is thrown but this is reported in #11825

[Duskfall]

Applied a simple fix for me in core. In private function del i added if(editor.readOnly) return false; Added also a unit test. Will commit in a bit