Add role ATK_ROLE_TREE_TABLE

Add role ATK_ROLE_TREE_TABLE

* atk/atkobject.h:
Add role ATK_ROLE_TREE_TABLE

* atk/atkrelation.h:
Remove ATK_RELATION_NODE_CHILDREN and ATK_RELATION_NODE_PARENT
and added ATK_RELATION_NODE_CHILD_OF; this is for use by TreeTable

* docs/tmpl/atk-unused.sgml, docs/tmpl/atkaction.smgl,
docs/tmpl/atkcomponent.sgml, docs/tmpl/atkdocument.sgml,
docs/tmpl/atkeditabletext.tmpl, docs/tmpl/atkimage.tmpl,
docs/tmpl/atkobject.sgml, docs/tmpl/atkrelation.sgml,
docs/tmpl/atkselection.sgml, docs/tmpl/atkstate.sgml,
docs/tmpl/atkstreamablecontent.sgml, docs/tmpl/atktable.sgml,
docs/tmpl/atktext.sgml, docs/tmpl/atkutil.sgml:
Update to reflect the current state of the documentation in the code.