Made GLX 1.3 the minimum supported version.
Made use of Win32 W-suffixed symbols explicit.
Further separation of window and context.

The context related parts of _GLFWwndconfig have been moved to
_GLFWctxconfig and given better names.  Window hint and attribute
members have been renamed to match.
Added missing link flags for threaded example.
Nessesery changes that include my parent window workaround.