TableView: implement an API to get the delegate item for a certain row/column

Currently you need to iterate through the contentItem of the table view to find the delegate items. And you will need to do this if you e.g want to resize a column based on the size of the items. Moreover, since row and column are context properties inside the delegate items, they cannot be queried from the outside. This forces you to create an extra set of properties/copies of the row and column inside the delegate to be used during the iteration.

So we need an API so that you can e.g call: TableView.itemAt(row, column)

But note that this needs a documented warning that the called should not store a pointer to the returned item, since it can be destroyed or reused at any time.

Also: consider a foreachItemInColumn(callbackFunction(item, row, column)