Sets the callback to call when you press the button.
Additional callback keys:
-armedPixmap pixmap file (CS)- 
Sets the pixmap file to use for the armed pixmap. 
The armed pixmap is displayed when you press a button.
 
clickCount- 
Number of multi-clicks pressed inside a button.
This is used to detect double or triple click events
inside a pushbutton.
 
value- 
Contains the pushbutton's label string.