Implemented Screenshot feature for Wpf browser.

Created separated pull request branch according to last comment of https://github.com/cefsharp/CefSharp/pull/446

Please note, that this feature won't work correctly unless https://github.com/cefsharp/CefSharp/issues/445 is fixed (either by merging WIP branch or https://github.com/ivan-sam/CefSharp/commit/cde1c0fa74d6817886ed9175cc2111251edcdfb9).