Split up toolbar button and icon classes

### I'm submitting a...
- [ ] Bug report
- [x] Feature request

While working on #41 I noticed it is necessary define button classes and icon classes in the same field, this is a result of changes made to have FontAwesome 5 compatibility.

This behaviour is confusing for developers working with the editor and error prone because the editor has to figure out which classes to add to the icon and which ones to add to the button.

I want to mark `className` as deprecated and introduce `iconClass` and `buttonClass`, those fields combined will facilitate the same functionality.