新增刪除子檢視控制器

要新增子檢視控制器：

- (void)displayContentController:(UIViewController *)vc {
   [self addChildViewController:vc];
   vc.view.frame = self.view.frame;
   [self.view addSubview:vc.view];
   [vc didMoveToParentViewController:self];
}

要刪除子檢視控制器：

- (void)hideContentController:(UIViewController *)vc {
   [vc willMoveToParentViewController:nil];
   [vc.view removeFromSuperview];
   [vc removeFromParentViewController];
}