Add limelight flash to the window that received focus.

[sg1705]

It'd be really nice if changing focus to a window can get a limelight flash to the border of the window. This will be very useful in dark mode where it is not easy to identify which window has focus.

I considered adding padding and margins to make selected window more prominent, but hasn't worked well.

This issue mentions using a limelight tool but I couldn't make it work propery.

[jvansant]

This would be very helpful.