Added xml documentation for the ContextHandle.
Made the casts between ContextHandles and IntPtrs explicit.
Updated all ContextHandle consumers to reflect the explicit cast.
Fixed vsync on X11.
Updated GraphicsContext interface, for better extensibility. Some public functions were moved to IGraphicsContextInternal.
Renamed DisplayDevice.PrimaryDisplay to DisplayDevice.Default.
Updated and documented new GameWindow constructors.
Improved GameWindow.Exit, added GameWindow.ExitAsync() and improved error handling.
Improved GraphicsContext and NativeGLWindow APIs (construction in constructor).
Made ContextHandle public.