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).
> 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?

How about we leave that out for now, can discuss it further at a later date if required?