Minor cleanup.
Moved the PropertyChangedExtensionMethods class, since it will be used from the WinForms example also.
Got rid of stuff which isn’t implemented. We can add it later if/when needed.
Removed unused usings.
#210. The WinForms WebView isn’t an IRenderWebBrowser; that interface is solely for OSR-based controls. So let’s make it work to use IWebBrowserInternal instead.