Feature Request - Add LoadPageAsync extension

[amaitland]

https://github.com/cefsharp/CefSharp/blob/cefsharp/83/CefSharp.OffScreen.Example/Program.cs#L127

Add to core framework as extension method.
Look at capturing sync context and continuing on calling thread.
Investigate better naming options for method.

Rough prototype at https://gist.github.com/amaitland/b027f827259bb2834a2879ee12cc6112

Pleaceholer issue. Details will be added shortly.

[amaitland]

Called when the loading state has changed. This callback will be executed twice -- once when loading is initiated either programmatically or by user action, and once when loading is terminated due to completion, cancellation of failure. It will be called before any calls to OnLoadStart and after all calls to OnLoadError and/or OnLoadEnd.

CefLoadHandler::OnLoadingStateChange should be called after OnLoadError which doesn't appear to be the case, to properly deal with errors we'll have to raise an issue upstream.