Win32: Add GLFW_WIN32_KEYBOARD_MENU

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

tests/opacity.c

@@ -59,6 +59,7 @@ int main(int argc, char** argv)
         exit(EXIT_FAILURE);
 
     glfwWindowHint(GLFW_SCALE_TO_MONITOR, GLFW_TRUE);
+    glfwWindowHint(GLFW_WIN32_KEYBOARD_MENU, GLFW_TRUE);
 
     window = glfwCreateWindow(400, 400, "Opacity", NULL, NULL);
     if (!window)