Function to be added to the list of functions to be called when an object receives focus.
Return: added focus tracker id, or 0 on failure.
Deprecated: This method is deprecated since ATK version 2.9.4. Focus tracking has been dropped as a feature to be implemented by ATK itself. If you need focus tracking on your implementation, subscribe to the state-changed:focused signal.
Adds the specified function to the list of functions to be called when an object receives focus.