title: constraints in prepareForInterfaceBuilder?
body: <p>I'm no great fan of <code>@IBDesignable</code>, but in playing around with it I find that if you add a subview in <code>prepareForInterfaceBuilder</code>, the subview is added but constraints that you apply to it are not obeyed.</p>
  
  <p>Is this a known limitation of <code>prepareForInterfaceBuilder</code>? That would make sense; I imagine that this method should be confined to doing things like giving a label some dummy text.</p>