[ListView] ListView.ListViewFocusModel#itemsContentListener is called twice

While looking into RT-39042 is saw that itemsContentListener is invoked 2 twice for an insert.

The reason is that:
a) the focus model itself registers it on the property via updateItemsObserver
b) itemsProperty in its invalidated property registers it via updateItemsObserver

2 solutions:
a) the focus model does not register it
b) updateItemsObserver also first removes the listener from the newList as well