Expose public API for refreshing widget mask and parts

[Dumluregn]

What is the purpose of this pull request?

New feature

Does your PR contain necessary tests?

All patches which 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 CKEditor 4 code style guide?

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

• PR is consistent with code style guide

What is the proposed changelog entry for this pull request?

*[#3775](https://github.com/ckeditor/ckeditor4/issues/3775): Widget [mask](https://ckeditor.com/docs/ckeditor4/latest/api/CKEDITOR_plugins_widget.html#property-mask) and [parts](https://ckeditor.com/docs/ckeditor4/latest/api/CKEDITOR_plugins_widget.html#property-parts) can now be refreshed dynamically via API calls.

What changes did you make?

I created two public methods: refreshMask() and refreshParts() aliasing accordingly setupMask() and setupParts() private methods. They needed to be slightly adjusted to handle the case in which one widget part was created at widget initialisation and the other one didn't (e.g. because it wasn't rendered yet). For that reason I also updated existing unit test for setting up parts.

Also in unit test there was an issue with IE8 that I tried to resolve for a few hours but still didn't find out why the first call of elementFromPoint() after refreshing parts and mask returns null and just added the second call.

Which issues your PR resolves?

Closes #3775.

[f1ames]

This PR introduces unexpected breaking change (~95 test fails on CI) - see review comments for more details.

[f1ames]

Looks good, just some changes in unit tests needed 👍

[f1ames]

LGTM, good job 👍

[f1ames]

Rebased onto latest major.