WinForms/WPF - Add ChromiumWebBrowser.UnregisterShutdownHandler

[amaitland]

Add a static method to unregister the Cef.Shutdown event handlers manually.

Currently you can set CefSharpSettings.ShutdownOnExit false, this can be a little tricky to know when and validate that it's been effecitve.

So we can add an explicit method to unregister the event(s).