effects/overview: Prevent WindowHeap accepting mouse events while searching

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.

BUG: 445707