gboolean iter_children ( GtkTreeModel* tree_model, GtkTreeIter* iter, GtkTreeIter* parent )
iter to point to the first child of
parent has no children,
FALSE is returned and
set to be invalid.
parent will remain a valid node after this
function has been called.
NULL returns the first node, equivalent to
gtk_tree_model_get_iter_first (tree_model, iter);