Fix for incorrect encoding of RGBA/HSLA colors in color dialog

[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?

* [#4351](https://github.com/ckeditor/ckeditor4/issues/4351): Fixed: Incorrect values for RGBA/HSLA colors in [Color Dialog](https://ckeditor.com/cke4/addon/colordialog).

What changes did you make?

I've unified functions used for color normalization in colorbutton plugin and added special handling for RGBA and HLSA colors.

Which issues does your PR resolve?

Closes #4351.

[Comandeer]

Rebased onto latest master.

[Dumluregn]

Fix is working and tests are passing 👍🏻

I've left mostly some refactoring proposals, please take a look.

[Comandeer]

Ok, I've fixed all issues with the test and refactored normalization method (by moving it to ColorBox.normalizeColo).

[Dumluregn]

Well done 🎉