Rework ResourceHandler

Includes a few changes around Resource Handling

- Split GetResourceHandler out of IRequestHandler into it's own IResourceHandler interface.
- Also include a `DefaultResourceHandler` implementation, so `LoadHtml` can easily be replaced with a `ResourceHandler`
- Update `WPF` and `WinForms` examples to reflect the changes.
- Add license header to `CefTerminationStatus` (unrelated commit)