Prevent adding transtions where states are not added

Current recursive adding of states does not work.

The old code tries to catch this,
```
  QState *parentState = state->parentState();
  if (parentState) {
    addStateOnly(parentState); // be sure that parent is added first
  }
```

but this does not work, because a unadded state could be used in addTransition(), then the graphs shows dangling states.