Sets iter to point to the previous node at the current level.
If there is no previous iter, FALSE is returned and iter is set to be invalid.
the [GtkTreeIter-struct|GtkTreeIter-struct]
TRUE if iter has been changed to the previous node
3.0
Sets iter to point to the previous node at the current level.
If there is no previous iter, FALSE is returned and iter is set to be invalid.