Issue with children-changed signal with flyweights

["Padraig O'Briain" <Padraig Obriain Sun COM>]

The signal handler for the signal "children-changed" on an AtkObject has
two parameters. The first is the index at which a child was added or 
removed and the second is the child which was added or removed, i.e.
an AtkObject.

This works adequately when there is a GtkWidget corresponding to the 
accessible object.

There are problems when the child is a cell in a TreeView, i.e. the child
being reported as added or removed is a flyweight. In this case we cannot
always return the flyweight as it may not have been created.

One option would be to return NULL for the AtkObject if the flyweight does not 
exist.


Padraig