effects/overview: Some search integration minor fixes
If the opacity property is set to 0, the item will still receive mouse events. One either needs to set the enabled or the visible property to false to ensure that the WindowHeap receives no mouse events.
The reason for setting the opacity to 0 instead of the visible property to false was that I thought later will nuke the paint node, but it seems like that's not the case.
The person that added krunner search integration uses vim. It seemed intuitive to that person that the Escape key should quit the search mode instead of quitting the effect. But it seems like more intuitive thing is to actually stop the effect.