title: Animation of UIStackView using isHidden
body: <p><strong>[EDIT]</strong>
  I've pushed a commit which contains a solution with zero height constraint, however, now I have to deal with implicit UISV constraints.</p>
  
  <p>I've spent hours to understand how to change an animation of appearing/disappearing items inside a stack view.
  I want to animate appearing of those two buttons from the bottom of the screen, however, they always fly from the top right corner:</p>
  
  <p><a href="https://i.stack.imgur.com/68KBt.gif" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/68KBt.gif" alt="Screencast"></a></p>
  
  <p>Is there any Cocoa way to configure the animation to start from the bottom?
  P.S. <a href="https://github.com/pomozoff/StackViewAnimation" rel="nofollow noreferrer">Link to the project</a>.</p>
  
  <p>Animation block:  </p>
  
  <pre><code>UIView.animate(withDuration: 5.0,
                     delay: 0.5,
                     options: .curveEaseInOut,
                     animations:
          {
              self.buttons.forEach { $0.isHidden = !$0.isHidden }
              self.stack.layoutIfNeeded()
          }, completion: nil)
  </code></pre>