The widget identified by child has been added to the receiver.
This method is public not because you can call it, but because
it can be useful to override it, not forgetting the call to
either the superclass implementation or #basicAddChild:, to
perform some initialization on the children just added. Answer
the new child.
basicAddChild: child
The widget identified by child has been added to the receiver.
Add it to the children collection and answer the new child.
This method is public because you can call it from #addChild:.