SharpDXElement Game should be XAML-defined and inheritable

I changed `SharpDXElement.cs` to fit inheritance and usage of XAML binding.
One may use `<SharpDXElement Game="AnyGameObject" />` now but I couldn't test it. It might ne wrong.
If it is not I would appreciate it if you implement this because I have to use it while I can't compile it.
Changed some things in GameWindowDesktopWpf aswell.