title: Make UITabBarController the root view controller
body: <p>I made an empty application, then later I and added a <code>UITabBarController</code> in the Storyboard. Then I checked it as the initial <code>viewcontroller</code> and connected two <code>NavigationControllers</code> to represent the tabs.
  But if I run the app now it says that "Application windows are expected to have a root <code>viewcontroller</code> at the end of application launch." and shows a white screen.</p>
  
  <p>My <code>AppDelegate</code> looks like this:</p>
  
  <pre><code>- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
  {
  self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
  self.window.backgroundColor = [UIColor whiteColor];
  [self.window makeKeyAndVisible];
  return YES;
  }
  </code></pre>
  
  <p>I have to add the <code>TabBarController</code> as the main window, but how to do this?
  If <code>TabBarController</code> was made programmatically I would have figured it but how to when it's created in storyboard? Can I make a property for it in <code>AppDelegate</code>? How to hook it in storyboard in that case?</p>