RootPresenter.RootView : add and use the protected method getAppRootPane...

Add and use the protected method RootPresenter.RootView#getAppRootPanel()

That method can be overriden to change what RootPanel to use and easily embed an app in an existing webpage.

Example of attaching the app to a panel with  id="appPanel" :

```java
public class MyAppRootPresenter extends RootPresenter {

    public static final class MyRootView extends RootView {
        @Override
        protected RootPanel getAppRootPanel() {
            return RootPanel.get("appPanel");
        }
    }

    @Inject
    public MyAppRootPresenter(final EventBus eventBus, final MyRootView myRootView) {
        super(eventBus, myRootView);
    }
}
```