Removed terrible hack for SDL_WINDOW_OPENGL

SDL2 will refuse to create an OpenGL context on a window with the SDL_WINDOW_FOREIGN flag (i.e. windows that are passed to SDL2 through SDL_CreateWindowFrom). This must be fixed upstream.
Improved resource disposal

Added IDisposable interface to all classes holding native data that must be freed. OpenTK.Toolkit.Init() now returns an IDisposable instance that can be used to cleanup all native data held by OpenTK. This is useful when re-initializing OpenTK (possibly in a new AppDomain), as is the case in the Example browser.
Clean up OpenTK after every example

Use the IDisposable instance returned by OpenTK.Toolkit.Init() to shutdown OpenTK after the end of each example run.