[Toolkit.Game] Start refactoring internals of GamePlatform/GameWindow to have a better plugin system and support multiple sub windows system (WPF/Windows Forms on Desktop, DrawingSurface/DrawingSurfaceBackground on WP8, IFrameview/SwapchainBackgroundPanel on WinRT)
[Toolkit.Game] Start to work on desktop refactoring for GameWindow
[Toolkit.Game] Add typed GameWindowContextWinForm and update all internal class in game