Defines UxmlTraits for the UXML root element.
| uxmlChildElementsDescription | Returns an enumerable containing UxmlChildElementDescription(typeof(VisualElement)), since the root element can contain VisualElements. |
| UxmlRootElementTraits | Constructor. |
| 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. |
| Init | Initialize a VisualElement instance with values from the UXML element attributes. |