WinForms/Wpf/OffScreen - Create specific CefSettings class for each version

- This is a breaking change as there is no common `CefSharp.CefSettings`, there is no `CefSharp.Wpf.CefSettings`, `CefSharp.WinForms.CefSettings` and `CefSharp.OffScreen.CefSettings`
 - Cef.Initialize has been removed, the `ChromiumWebBrowser` constructor will call the `Cef.Initialize` with the defaults for your flavour
 - CefSettings.FocusedNodeChangedEnabled has been moved to CefSharpSettings.FocusedNodeChangedEnabled

Resolves #2477