Answer a DirectedMessage that is sent when the receiver is modified,
or nil if none has been set up.
callback: aReceiver message: aSymbol
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.
Set up so that aReceiver is sent the aSymbol message (the name of
a one- or two-argument selector) when the receiver is clicked.
If the method accepts two argument, the receiver is passed
together with anObject; if it accepts a single one, instead,
only anObject is passed.