$model->iter_next fix

["Ross McFarland" <rwmcfa1 neces com>]

until now:

$nextiter = $model->iter_next ($iter)

modified $iter and then returned a copy of it. which was likely unexpected and
unclear behavior. it could/would cause problems when used in
$model->foeach'es. in c iter is pass-by-reference and modified. to not have
the problems, in c, you would simply copy the iter and the iter_next the copy,
but you don't get copy in perl for various/obvious reasons.  in Gtk2 iters are
returned rather than modifying a passed parameter (very un-perl-like) this
function tried to do that, but didn't quite get there. it now does.

this is now fixed in head of cvs. someone found this and came on irc last
night about 2 in the morning, i don't know who it was (real name) so no credit
was given if you're on here and you want credit let me know.

-rm