title: Forcing a tri-state checkbox to not move to indeterminate state
body: <p>I'm working in WPF and i have an interesting requirement.
  I need my checkboxes to be ThreeState, so if only some of the child elements are selected it shows as indeterminate. But when a use clicks it, i want it to select either true or false.</p>
  
  <p>Here is a story to represent my requirements:</p>
  
  <pre><code>item - indeterminate  
      subItem - checked  
      subItem - unchecked  
      subItem - checked  
  </code></pre>
  
  <p>When a user clicks <code>item</code>, the checkbox should alternate between checked and unchecked. The user should never be able to select 'indeterminate' as a state. Is this possible?</p>