addSetMembersModifiedCallback(node, function, clientData=None) -> id
Registers callbacks for set modified messages.
* node (MObject) - the set that has triggered a setModified event
* function (MMessage::MNodeFunction) - the callback function
* function - callable which will be passed a MObject indicating the
set that has triggered a setModified event and the clientData object
* clientData - User defined data passed to the callback function
* return: Identifier used for removing the callback.