[Enhancement] BoxView handlers should use Paint

## Summary
We added #2250 but we used Color instead of Paint for now, we should implement the handler using the `Paint` type so we can do gradients etc

## API Changes

```
public interface IBoxView : IView
{
	Paint Fill { get; }
}
```

## Intended Use Case

Allow to have a box with gradient paint or future kinds of `Paint`
