The method called by this callback receives the VisualElement to bind, and the index of the
element to bind it to.
If this property and makeItem are not set, Unity will try to bind to a SerializedProperty if
bound, or simply set text in the created Label.
Note:: Setting this callback without also setting unbindItem might result in unexpected behavior.
This is because the default implementation of unbindItem expects the default implementation of bindItem.