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.