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
%TRUE if @iter has been changed to the previous node
3.0
See Implementation
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.