effects/overview: Layout window thumbnails in updatePolish()

Currently, ExpoLayout delays layout updates by using a singleshot timer
(primarily due to the author of the effect not being aware of
QQuickItem::polish() and QQuickItem::updatePolish()).

This change makes ExpoLayout schedule layout updates using QtQuick's
native item polish machinery, which gets triggered before rendering and
thus we can batch more geometry updates.

In addition to that, this change simplifies the initialization code in
ScreenView by making the fact that ExpoLayout is arranged right before
rendering internal to the WindowHeap type.