title: Animator of NSView does not animate alpha value change
body: <p>I've got a problem handling the animators of NSViews. 
  In the code below I create a (custom) controller which also has got a view. I want it to fade it into the window, and fade the old one out. But it doesn't animate at all, the new view just appears, and the old one gets "removeFromSuperview" instantly. 
  Also, I have seen that the old view behaves normally, it fades out. But the new one is in the way and doesn't fade at all.
  My code:</p>
  
  <pre><code>LTController *newController=[[LTController alloc] init]];
  
  [[newController view] aFrame];
  
  [[newController view] setAlphaValue:0];
  [[[self window] contentView] addSubview:[newController view]];
  
  [[[newController view] animator] setAlphaValue:1];
  [[[viewController view] animator] setAlphaValue:0];
  
  [viewController view] performSelector:@selector(removeFromSuperview) withObject:nil afterDelay:[[NSAnimationContext currentContext] duration]];
  
  [self setViewController:newController];
  </code></pre>
  
  <p>How can it be that the animator doesn't do anything to animate? What am I doing wrong?</p>
  
  <p>Thanks in before,</p>
  
  <p>Ivorius</p>