atom: Add a GDATA_LINK_PARENT define

This is a link type which is included in the GData specification (in the
RelaxNG schema), but is not documented anywhere. Good job, Google. Best
for us to define it and allow clients to use it, than to ignore it.

New API:
 • GDATA_LINK_PARENT

https://bugzilla.gnome.org/show_bug.cgi?id=725827
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