[Toolkit.WP8] DrawingSurface support

Added support for DrawingSurface on WP8 platform.
I did not fully understood the implementation for DrawingSurfaceBackgroundGrid - so I am not sure if my implementation is correct.
Please confirm that the general approach is fine, then I will refactor the classes GameWindowWindowsPhoneXaml and GameWindowWindowsPhoneBackgroundXaml to reduce code duplication.