add registerVisibleHandler(HandlerRegistration) for PresenterWidget (patch)

Hi,

it would be nice if ```PresenterWidget``` would have a method 
```
registerVisibleHandler(HandlerRegistration)
```

There is already a similar functionality (```addVisibleHandler```), which requires an event type as argument. My suggested method would take any previously created ```HandlerRegistration```. A similar method ```addRegisteredHandler``` is already presenter, which removes the handlers in the ```onUnbind``` method. However, it might be more efficient to remove a handler already in the ```onHide```.

The functionality is already there in the code using the field ```visibleHandlerRegistrations``` of the ```PresenterWidget```. It is just not accessible on a protected/public level. 

Thanks
Daniel

`#onHide()`