Added Css Classes field to wp-link in editor.

[mikhailroot]

Good day to WP Core developers!

Currently the only way to add css class to a link in wysiwyg is to switch editor to textual mode and add class attribute by hand.
So I've decided to add this field to wp-link editor dialog. It's helpfull when you use link as a button and you definitely don't want to style all content links as buttons, so you are able to edit class link attribute in wp-link editing dialog which is much user friendly.

[mikhailroot]

Patch with css classes editor field for wp-link implementation.

[pento]

Thank you for the suggestion and patch, @mikhailroot!

For the classic editor, I think this is best served by a plugin, such as ​WP Edit.