[vtkusers] Callback implementation

[masabumi ishihara]

Hi VTK Users,

We are tring to develop new viewer.
On the process of the development, we realized that we need to
customize the callback method because the events from the mouse or
keyboard have to bring about the respective responses.
Here, we would like to focus the problem on vtkImageViewer2.
In vtkImageViewer2.cxx, there is vtkImageViewer2Callback class for
event customization.
So, is it good for us to impliment only vtkImageViewer2Callback class
to customize new class  for the same response as
vtkImageViewer2Callback?
Is there any other necessary metod or class to impliment for the same
response as vtkImageViewer2Callback?

Thanks,

Maty