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.