A 
Callback is an arbitrary action to be fired - usually passed into a  method to be fired asynchronously as
 a notificaction of some event.
 The 
callback can be defined in the following formats:
 - a
 function
- A string containing an expression to evaluate
- An object with the following properties:
 - target: fire in the scope of this target - when the action fires,       the target will be available asthis.
 - methodName: if specified we'll check for a method on the target object with this       
 name.
 
Callbacks are fired via the  Class.fireCallback() method, which allows named
 parameters to be passed into the callback at runtime. If the Callback was specified as a string of script, these
 parameters are available as local variables at eval time.
 For specific Smart GWT methods that make use of
 
Callback objects, see local documentation for information on parameters and scope.