Emit ChildrenChanged signal on hierarchy updates

We currently do not notify ATs of changes in the children of an accessible object, which means that they can only represent the current hierarchy.

• emit the atspi.Event.Object.ChildrenChanged signal whenever a new accessible object is added or removed from its parent.
• needs to monitor the toplevel listmodel to get notification about toplevels appearing and disappearing
• special-case the stack page case to avoid emitting a child-added signal for what ends up being a grandchild