Adds the state of the specified type to the state set if it is not already
present.
Note that because an atk.StateSet is a read-only object, this method should
be used to add a state to a newly-created set which will then be returned by
atk_object_ref_state_set It should not be used to modify the existing state
of an object. See also atk_object_notify_state_change
Adds the state of the specified type to the state set if it is not already present.
Note that because an atk.StateSet is a read-only object, this method should be used to add a state to a newly-created set which will then be returned by atk_object_ref_state_set It should not be used to modify the existing state of an object. See also atk_object_notify_state_change