Unify color handling

[Comandeer]

Type of report

Task

Provide description of the task

See #4351 (comment). Currently we are handling colors in a very convoluted way, with normalizing hexes without hashes and then adding hashes in several places. Probably we should rewrite the whole color handling logic to use CKEDITOR.tools.color.

Other details

• Installed CKEditor plugins: colorbutton, colordialog

[Comandeer]

Proposed changes/additions to the CKEDITOR.tools.color:

• isValidColor should be a public property (and probably should be called just isValid);
• checking the validity of the color sounds like a good candidate for a static method,
• there should be support for hex-like values (fff and ffffff) as we use them everywhere.

[f1ames]

We had started working on the PR to cover this issue but decided to postpone it due to #4718 (comment).