glXGetProcAddress may return a non-null value even if a function is not
supported by the server. We need to check the extension string using
any GLX extensions. Fixes issue http://www.opentk.com/node/3111 "GLX
extension support is not checked correctly".
Furthermore, mode selection is now performed explicitly by the
X11GLContext constructor.
Added OpenGL 3.0 support for GLX contexts.
Added support for creating dummy contexts (useful if you are creating contexts from an external library).
Cleaned up unused and stale code.
Added xml documentation for the ContextHandle.
Made the casts between ContextHandles and IntPtrs explicit.
Updated all ContextHandle consumers to reflect the explicit cast.