title: How to override navigation bar buttons
body: <p>Is it possible to override all the buttons in the Android navigation bar?
  Is it maybe possible to add a new button?</p>
  
  <p>How can I change the appearance of the buttons? I want to set an other background bitmap.</p>
  
  <p>I am using a Nexus 7.
  I just know onBackPressed() but what about the other two buttons?
  And how are they called?</p>