Defines UxmlTraits for the BindableElement.
| BindableElement.UxmlTraits | Constructor. |
| Init | Initialize EnumField properties using values from the attribute bag. |
| canHaveAnyAttribute | Must return true if the UXML element attributes are not restricted to the values enumerated by uxmlAttributesDescription. |
| uxmlAttributesDescription | Describes the UXML attributes expected by the element. The attributes enumerated here will appear in the UXML schema. |
| uxmlChildElementsDescription | Describes the types of element that can appear as children of this element in a UXML file. |
| focusable | The focusable attribute. |
| focusIndex | The focus index attribute. |
| uxmlChildElementsDescription | Returns an enumerable containing UxmlChildElementDescription(typeof(VisualElement)), since VisualElements can contain other VisualElements. |