Suggestion: saving/loading callback

Would be nice if you could add some kind of unique callback when saving/loading.
I simpy added this at the end of ofxControlPanel::loadSettings :

```
guiCallbackData hackcallback;
hackcallback.addValueS("LOADING_UNIQUE_CALLBACK_HACK");
ofNotifyEvent(guiEvent, hackcallback, this);
```

 and it got me the functionality I needed. Great code by the way!
