1.12.2 BLOX.BDropDown: callbacks
- Answer a DirectedMessage that is sent when the receiver is clicked,
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.
- Generate a synthetic callback