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, iter is set to an invalid
iterator and false is returned. parent will remain a valid node after this function has
been called. As a special case, if parent is null, then the n-th root node is
set.
| this |
a `GtkTreeModel` |
| iter |
the `GtkTreeIter` to set to the nth child |
| parent |
the `GtkTreeIter` to get the child from |
| n |
the index of the desired child |
|
true, if |