Callback for constructing the VisualElement that is the template for each recycled and re-bound element in the list.
This callback needs to call a function that constructs a blank VisualElement that is
bound to an element from the list.
The BaseVerticalCollectionView automatically creates enough elements to fill the visible area, and adds more if the area
is expanded. As the user scrolls, the BaseVerticalCollectionView cycles elements in and out as they appear or disappear.
If this property and bindItem are not set, Unity will either create a PropertyField if bound
to a SerializedProperty, or create an empty label for any other case.