title: Navigate to parent of parent stack
body: <p>I have this scenario where i have a <code>StackNavigator</code> nested in a <code>TabNavigator</code> nested in another <code>StackNavigator</code>.</p>
  
  <pre><code>const TabOneStack = StackNavigator({
    ScreenA: { screen: ScreenA },
    ScreenB: { screen: ScreenB }
  });
  
  const MainTabs = TabNavigator({
    TabOne: { screen: TabOneStack },
    TabTwo: { screen: TabTwoStack }
  });
  
  const Root = StackNavigator({
    HomeScreen: { screen: HomeScreen },
    MainTabs: { screen: MainTabs }
  });
  </code></pre>
  
  <p>Everything works but i cant figure out how to navigate for example from <code>ScreenA</code> back to the Home screen in the root <code>StackNavigator</code>.
  After the <code>HomeScreen</code> the User navigates directly to ScreenA.
  The back button in the header in ScreenA works fine and brings me back to Home but need a way to have a button that brings me back to the HomeScreen.
  <code>this.props.navigation.goBack()</code> does not work unfortunately.</p>
  
  <p>i also tried</p>
  
  <pre><code>const backAction = NavigationActions.reset({
    index: 0,
    key: null,
    actions: [
      NavigationActions.navigate({ routeName: 'HomeScreen'})
    ]
  });    
  this.props.navigation.dispatch(backAction));
  </code></pre>
  
  <p>but i get:</p>
  
  <p><strong>There is no route defined for key HomeScreen. Must be one of: 'ScreenA', 'ScreenB'.</strong></p>
  
  <p>What is the correct way to do this?</p>