Win32: Add GLFW_WIN32_KEYBOARD_MENU

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

tests/inputlag.c

@@ -202,6 +202,7 @@ int main(int argc, char** argv)
     glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 0);
 
     glfwWindowHint(GLFW_SCALE_TO_MONITOR, GLFW_TRUE);
+    glfwWindowHint(GLFW_WIN32_KEYBOARD_MENU, GLFW_TRUE);
 
     window = glfwCreateWindow(width, height, "Input lag test", monitor, NULL);
     if (!window)