Properly invalidate iterators in iter_next, iter_children,
iter_nth_child and iter_parent when FALSE is returned. ListStore was
actually already mostly in order as the GSequence pointers are very
strictly checked, but to be a little stricter we've added stamp = 0
lines. TreeStore did not invalidate its iterators, so stamp = 0 lines
were added where appropriate. Unit tests have been added to check this.
Fixes bug 586374, reported by Mike Gemunde.