The signal "children-changed" is emitted when a child is added or removed form an object.
It supports two details: "add" and "remove"
The index of the added or removed child. The value can be -1. This is used if the value is not known by the implementor when the child is added/removed or irrelevant.
A gpointer to the child AtkObject which was added or removed. If the child was removed, it is possible that it is not available for the implementor. In that case this pointer can be NULL.