bugfix: List editor behaves wrongly when calling setValue() more than once

It was accumulating elements on each call, apart of that, the render() method also wasn't refreshing the DOM after the first call.

The way to reproduce is simple..
```
// given an exisiting and already rendered form...
form.setValue({ a_list_propery: ['a', 'b'] });
```
The form was not only accumulating elements, but also not refreshing the view.

I hope this could reach the trunk,
Thanks,