Improve additional classes field

[coreyworrell]

What problem does this address?

Makes adding custom classes more user friendly for styling options that may not fit into the other options offered by Gutenberg already.

What is your proposed solution?

Allow user to select from available classes that the theme offers along with adding custom. Similar to how EditorsKit plugin does it by using a token field.

Having the theme provide (via JSON and/or PHP) a list of classes (with labels) and possibly by group could make this very easy.

Utility classes for padding, margin, maximum widths, etc could take advantage of this. A label could be used with the underlying value being the class itself.

[CreativeDive]

@coreyworrell This is a fantastic idea !

[mtias]

There are several items for iteration on style variations that relate to the class field in #7551. Would you mind transferring this request there?

[coreyworrell]

@mtias okay done. thank you.