add window-inactive pseudo-class

[yololippy]

I see this feature on MDN

The :-moz-window-inactive CSS pseudo-class is a Mozilla extension that matches any element while it's in an inactive window.

But I test this feature on chrome 66 which support this, but it only be use with other pseudo-class, such as ::selection:window-inactive, ::-webkit-scrollbar-thumb:window-inactive. So I think it would be great if can i use provides this feature

[zhangenming]

ping it.