Scene's `onMouseDrag*` setters have incorrect type parameters, `jfxsi.MouseEvent` instead `jfxsi.MouseDragEvent`.

For instance, in `Scene` we have

``` scala
def onMouseDragEntered_=(v: jfxe.EventHandler[_ >: jfxsi.MouseEvent]) {...}
```

but should be

``` scala
def onMouseDragEntered_=(v: jfxe.EventHandler[_ >: jfxsi.MouseDragEvent]) {...)
```
