Answer a DirectedMessage that is sent when the receiver is clicked, or nil if none has been set up.
Set up so that aReceiver is sent the aSymbol message (the name of a zero- or one-argument selector) when the receiver is clicked. If the method accepts an argument, the receiver is passed.
Generate a synthetic callback
This is the default callback for the widget; it does nothing if you don’t override it. Of course if a subclass overriddes this you (user of the class) might desire to call this method from your own callback.