Enhancement and Minor api changes take 2

I've been working on enhancing the WinForms example to support Tabbed Browsing, as part of that process these are the changes and enhancements that I'd like to see made. Completes a few missing pieces in terms of WinForms support.

- Add Print(), Back(), Forward() to `IWebBrowser`
- Remove unused NUnit Reference
- Remove IsLoading, IsBrowserInitialized, TooltipText public setters as they should never be set from user code
- Remove redundant event delegates and replace with generic EventHandler<> (built into .Net core libs)
- Add IsLoading and IsBrowserInitialized event handlers to WinForms, should now bring the events in line with the WPF version.
- Improved Xml comments in `IWebBrowser`
- Minor ReSharper inspired cleanups

Feedback is most welcome!