Fix polishItems bug

updatePolish is not called for items which are not visible.
However if a polish is scheduled, updatePolish has to be called when the item
becomes eventually visible.

This patch makes sure, that invisible items are not removed from the itemsToPolish list.

Change-Id: I1ad929dc6de8e61edbd1c4df88ae5bc951979ff1
Reviewed-by: Ulf Hermann <ulf.hermann@digia.com>
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>