Extending functionalities of buttons

Hi everyone ! This issue is a feature request to trigger discussions about buttons in traitsui views. It would be nice if UIs could have buttons with various states, which would change their style. More specifically:

- [x] changing dynamically the icon of a button: the same way it is possible to pass a trait to `ButtonEditor` to dynamically change the label of a button, it would be great to pass a trait representing a `pyface.ImageResource` so we could also change the icon of the button. (#1566)
- [ ] being able to keep the button in a pressed state: from an idea of @corranwebster , this could simply be another style for `BooleanEditor`: instead of displaying a checkbox or a text field, this editor could render a button, pressed or not depending if the rendered boolean is true or false. Otherwise, `ButtonEditor` could feature a trait we could pass to set the button state to be pressed or not.