title: Don't change NSButton appearance when pressed or disabled
body: <p>I have two NSButtons with images for both their on states and off states. Only one should be active at a time; click one and then click the other to change a property back and forth.</p>
  
  <p>The problem is that if I disable a button when it's clicked so it cannot be clicked again, then the image is dimmed when the button is disabled--and I don't want it dimmed, I just want to use the alternate image.
  On the other hand, if I just leave the button enabled, but programmatically just don't run any code when it is clicked, then there's a flashing effect as the mouse clicks--which is distracting, when the button should not do anything.</p>
  
  <p>So I either need to prevent the button from being dimmed when it is disabled, or prevent it from changing the button appearance while the mouse button is held down.</p>
  
  <p>After reading up, it sounds like I need to subclass NSButtonCell and override - (BOOL)imageDimsWhenDisabled to do the former. But I can't figure out exactly how to subclass it (what sort of NSButtonCell class I should inherit from) and if the "setCell" method of NSButton is enough to use the new NSButtonCell class, or if I need to subclass NSButton as well.</p>
  
  <p>Some tips on that would be appreciated, or perhaps there's a completely different approach that would achieve my objectives.</p>