fix: copy ItemOwnZValue when cloning item

[letsfindaway]

This PR should fix #1167

• for most items, only zValue was copied when cloning
• now also copy the ItemOwnZValue data property

Notes:

• It could also be an idea to use UBGraphicsItem::assignZValue() in these cases. which assigns a z value to both, the data property and the item itself. However I'm not sure whether these values could differ while cloning.
• As the z value is something common to all items, we could also extract copying of this parameter to a base class. However I did not want to introduce such a change here without discussion. There are possibly other such common parameters which should be copied for all item types.