Implement CefBrowserHost::SetFocus for WinForms

In our app we need to programmatically set focus to the web browser, but we found it wasn't possible (`Control.Focus()` doesn't do it).  This implements `SetFocus` so we could call `SetFocus(true)`, which does work (input focus moves to the browser).
It is a bit bad.  I wasn't positive about this part of the change.

If someone has a `ChromiumWebBrowser` instance and calls `Focus()`, this implementation does get called and the browser gets focus.  But if someone has a `Control` instance, it won't get called and the focus won't change.

So for example, if someone has a utility function like this that they pass a browser into as part of a list...
```
public void FocusLastControl(List<Control> controls)
{
    controls[controls.Count - 1].Focus();
}
```

... then it won't work (browser won't get focused).

I could just as easily leave this out so `Focus()` never works and doesn't give false hope to users, and they are forced to call `SetFocus()` which will always work.  What do you think?