title: NSButton without "click highlighting"
body: <p>I've got an <code>NSButton</code> with an image (and no text), over a blueish background.
  The Style is "<code>Square</code>", type is "<code>Momentary Push In</code>", not bordered etc.</p>
  
  <p>When I click the button, there is a light rect around it, which is highlighted for a second. How do I get rid of it?</p>
  
  <p>I only want to have the image displayed and when clicked there shouldn't be any highlight indication… Should I subclass the button or is there a simple setting in Interface Builder?</p>
  
  <p>Not clicked – normal state: <img src="https://i.stack.imgur.com/7Pmgh.png" alt="NSButton normal state"></p>
  
  <p>Clicked - highlighted: <img src="https://i.stack.imgur.com/OMQEV.png" alt="highlighted NSButton when clicked"></p>
  
  <p>so I don't want that highlighting rect ;-)</p>