iterNthChild
Sets @iter to be the child of @parent, using the given index.
The first index is 0. If @n is too big, or @parent has no children,
Return
true, if @parent has an @n-th child
Parameters
iter
the GtkTreeIter
to set to the nth child
parent
the GtkTreeIter
to get the child from
n
the index of the desired child