The #AtkFocusHandler to be attached to @component
Return: a handler id which can be used in atk_component_remove_focus_handler() or zero if the handler was already added.
Deprecated: If you need to track when an object gains or lose the focus, use the #AtkObject::state-change "focused" notification instead.
Add the specified handler to the set of functions to be called when this object receives focus events (in or out). If the handler is already added it is not added again