Add option for windows going grayscale when unfocused

[NTICompass]

I created the issue #4258 before realizing that this was a feature added in 1.6b.

Browser background now goes grayscale when unfocused

Can we have an option to toggle this feature on and off? I like my colorful Zen window. Also, it makes it a little hard to tell which unfocused window is "normal" and which is "private browsing" at a glance.