Unloaded disposes Window

[code-beans]

The Loaded and Unloaded events are frequently used when embedding the WpfSharpDxControl in other controls. Right now invoking the Unloaded event will also invoke Dispose() on the WpfSharpDxControl thus terminates the Win32 Window and frees any resources (device,rendertarget,etc.).
When the Loaded event is invoked again (by other parent controls) and the Rendering=true flag is set rendering will throw exceptions.