Use editor's title for new preview window

[Elyasin]

What is the purpose of this pull request?

Other: It is a PR for the Issue 4026

Does your PR contain necessary tests?

No, it does not.

1. I am not sure if this change requires a test.
2. bender exits with a failure. I tried to follow the steps described in the corresponding documentation (Setting up tests, run tests, etc.). Too time consuming for this simple test.

This PR contains

• None, as explained above

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.

• [X ] PR is consistent with the code style guide

What is the proposed changelog entry for this pull request?

* [#4026](/~https://github.com/ckeditor/ckeditor4/issues/4026): Preview addon should use editor's "title" property for the title of the new window

What changes did you make?

Changed title = editor.lang.preview.preview, to title = editor.title,

Which issues does your PR resolve?

Closes #4026

[Comandeer]

Hi, @Elyasin! Thanks for your contribution.

You're right, the change is a simple one, however, it still requires tests (ideally both automatic and manual ones). Do you want to devote some more time to create them or should I add them to this PR?

[Elyasin]

Hi @Comandeer - I tried to add a test. Unfortunately I could not get it running without errors. I followed the description about setting up testing and how to execute but bender errors out before even running the tests. I would need some help as I could not figure it out. I will try again and let you know soon.

[Elyasin]

I added an automatic test. I don't understand how manual tests word. Unfortunately it is not well explained. All I could understand is that they exist and there is a folder with two files for manual test in the preview test folder.

I decided to leave it as is for now. I'd need more support otherwise.

[Comandeer]

@Elyasin, thanks for your contribution!

The unit test seems ok, I've just slightly adjusted it (I moved canceling event before resume to ensure it would be canceled even if the assertion failed and replace assert.isArray with assert.isMatching as it's an assertion dedicated to checking against regexes). I've also added a manual test.

As for manual tests, I'd like to think of them as tests intended for humans (where unit ones are for machines). The description of the testing procedure (e.g. what to click in the editor's toolbar and what to expect after the click) is put in the *.md file. The code that initializes the editor is in the *.html file. But you're right, it could be described more in our docs – I'll add an issue for it.