effects/overview: Make the window heap cover the maximize area

Currently, window thumbnails may be placed behind the panel, which doesn't look good.

With this, the window heap will cover the same area as the maximize area.

Draft as I ain't sure about the final design. It also makes sense to have the expoarea as an item. That way, we could use anchors, but qml code won't be in charge of positioning of the expoarea itself...