title: Multiple windows per document (Cocoa document based app)
body: <p>Let's say I have created a new Cocoa application and use <code>Document-based application</code> when I create the project. It will have one window, the one from the <code>NSDocument</code> subclass. How can I make it so that two (or more) windows belong to each document?</p>
  
  <p>I have created an <code>NSWindowController</code> subclass with a .xib file, where I have created the interface. How can I show this window? And how does communication between the <code>NSWindowController</code> subclass and the <code>NSDocument</code> subclass work?</p>
  
  <p>(I use core data, so it is really an <code>NSPersistentDocument</code> subclass, but I don't think it matters for this particular question.)</p>