Ensure highlight effect is cancelled after activating a window
Otherwise after activating by clicking on thumbnail, it gets stuck in a weird highlight effect state. For example, Yakuake won't open properly.
Also fixes problem where the clicked thumbnail remains "hovered", and so a window gets highlighted whenever the tooltip appears, even if you didn't hover over a thumbnail.
Should be cherry-picked to 5.22.
Edited by Bharadwaj Raju