Feature: whole word option

As requested in #195

Many things are open to suggestions, like the option button label or the key bindings.

- [x] Implement the whole-word search.
- [x] Handle serialization of the whole-word option state.
- [x] Add an option button and adjust styles of the group so that it can fit in it.
- [x] Display the whole-word option state in the label.

Don't hesitate to comment if you see anything that is missing.