Bibtex input:  Allow xref to be nested.

Example:

@xdata{macmillan:name,
    Publisher = {Macmillan}}

@xdata{macmillan:place,
    Location = {New York and London}}

@xdata{macmillan,
    Xdata = {macmillan:name, macmillan:place}}

@book{item1,
    Author = {Author, Ann},
    Date = {2012},
    Title = {Title},
    Xdata = {macmillan}}