Song Support for WinDX

Adding of the rather complex MediaSession song support for the Windows desktop DirectX platform.

 - Implemented play/pause/stop/resume/mute/volume for Song.
 - Made a few non-XNA Song members internal.
 - The Song.FilePath is now absolute for desktop & WinRT systems.

The end of song events are not currently working... I need to get a fix into SharpDX before I can correct that still.