ClutterListModel has been added as a terminal class in case we
decided to change API or implementation.

Apparently, it's a lot more resilient than we expected -- or far
too few are using it and reporting bugs. Once common issue, though,
is that it cannot be subclassed, hence it's fairly limited in its
usage.

In the hope that more developers will start using it, here's a
patch that makes ListModel a fully subclassable object.

May whatever god you believe in have mercy on your eventual soul.