Update Listbox EditLine and EditBox to use style - not ready for merge

Hi all,

Same as usual, mostly minor updates on this one. I changed the way `list_box` works a little bit to match Shoes 3 (and the manual) better. More about that in line.

As for the failing specs, I'm hoping someone can help me there. All of the `editable_element` [specs](https://github.com/shoes/shoes4/blob/master/spec/shoes/shared_examples/dsl/editable_element.rb) are failing. I can't tell why though. When I do the same tests manually they seem to pass:

```ruby
Shoes.app do
  e1 = edit_line
  e2 = edit_box

  p e1.text == ''
  p e2.text == ''
  
  e1 = edit_line 'Hello text here'
  e2 = edit_box 'Hello text here'
  
  p e1.text == 'Hello text here'
  p e2.text == 'Hello text here'
end
```

Otherwise I think this one is ready to go. I look forward to your comments / suggestions (even if they take a little while to get here ;) 
I moved `secret?` here since `edit_box`s can't be secret.