Fix CKEDITOR.inlineAll inlining also elements with editor already attached to them

[Comandeer]

What is the purpose of this pull request?

Bug fix

Does your PR contain necessary tests?

All patches that change the editor code must include tests. You can always read more
on PR testing,
how to set the testing environment and
how to create tests
in the official CKEditor documentation.

This PR contains

• Unit tests
• Manual tests

Did you follow the CKEditor 4 code style guide?

Your code should follow the guidelines from the CKEditor 4 code style guide which helps keep the entire codebase consistent.

• PR is consistent with the code style guide

What is the proposed changelog entry for this pull request?

* [#4293](https://github.com/ckeditor/ckeditor4/issues/4293): Fixed: [`CKEDITOR.inlineAll`](https://ckeditor.com/docs/ckeditor4/latest/api/CKEDITOR.html#method-inlineAll) tries to inline also elements with editor already attached to them.

What changes did you make?

I've just added a simple check if the element has an attached editor, using element#getEditor().

Which issues does your PR resolve?

Closes #4267.
Closes #4293.

[Dumluregn]

LGTM! 🎉

✅ Chrome
✅ Firefox
✅ Safari
✅ Edge
✅ Opera
✅ IE11
✅ IE10
✅ IE9
✅ IE8

I'll do just some minor polishing.