let Item::clone() create a deep copy of the item tree

Product multiplexing uses Item::clone() to create a product item
per profile. Cloning the product is not enough, however.
We must create clones of the children and item properties too.

Task-number: QBS-690
Change-Id: I8f46488ffa5df255ee9c2b0571145c7ff034c204
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>