Inserts an indexed child to the node. Shifts the element currently at that position, if any, and any subsequent elements to the right (adds one to their indices). Children may be added to the end of the list of indexed children with the second version of addIndexedChild().
Unlike many other APIs, adding a child does not remove it from any other parent nodes. In fact, the same child UINode instance is allowed to appear in multiple different indices of the same parent UINode.
@param childIndex the zero-based index to add the child at.
@param child the new child node
@see #removeIndexedChild
@see #clearIndexedChildren
@see java.util.List#add