Win32: Add GLFW_WIN32_KEYBOARD_MENU

This platform specific window hint enables access to the Windows window
menu via the keyboard shortcuts.

src/internal.h

@@ -274,6 +274,9 @@ struct _GLFWwndconfig
         char      className[256];
         char      instanceName[256];
     } x11;
+    struct {
+        GLFWbool  keymenu;
+    } win32;
 };
 
 // Context configuration