[Mac] fix color chooser sample

Color chooser sample is broken on Mac. There are 2 separate problems. 

1. Color chooser uses XwtWidgetBackend instead of a native backend, so trying to cast the backend to a ViewBackend throws and exception. I added the method GetNativeBackend() to recursively get a ViewBackend object in the case of an XwtWidgetBackend object.

2. After fixing the above problem, there is no more exception thrown, but the color chooser is still not visible. It turns out you need to call SetAutosizeMode (true) in order for it to be visible, so I added code to set this for all XwtWidgetBackend objects.