This function returns the name of the file at index idx in the xad archive specified by
id.
To find out the number of files in a xad archive, you can query #XADATTRNUMENTRIES with
Hollywood's GetAttribute() function. See xad.GetObjectType for details.
Inputs
id
identifier of the xad archive to use
idx
index to query (in the range of 0 to number of entries minus 1)