addcallback

Add a function handle to a callback list. All functions in the list are run when the callback execut

Has the same functionality as IPTADDCALLBACK, but more:
- does not need image processing toolkit
- easier removal of callback functions
- allows passing arguments to the callback functions
- clean & transparant: contains the function LISTCALLBACK to view all callback functions

ADDCALLBACK( H, CALLBACK, FUNC_HANDLE )
adds the function handle FUNC_HANDLE to the list of functions to be called when the callback specified by CALLBACK executes.

REMOVECALLBACK( H, CALLBACK, FUNC_HANDLE )
removes all callback functions (for the specified callback) that have the specified function handle.

For compatibility, one can also use:
REMOVECALLBACK( H, CALLBACK, ID )

LISTCALLBACK
Gives the complete lists of all callback functions, sorted by object and callback.