title: Checked Checkbox should be not uncheckable
body: <p>I have some difficulty settings in a game. Because only one option should be active at once i did e.g. for option 'Easy'</p>
  
  <pre><code>//Uncheck all other difficulty options
  if(ui-&gt;actionMedium-&gt;isChecked())
  {
      ui-&gt;actionMedium-&gt;setChecked(false);
  }
  
  if(ui-&gt;actionHard-&gt;isChecked())
  {
      ui-&gt;actionHard-&gt;setChecked(false);
  }
  </code></pre>
  
  <p>Now the problem is that if i click on an already checked checkbox the checkbox will uncheck. Ok, thats the normal behaviour of a checkbox, so i added</p>
  
  <pre><code>//check if its alreay checked
  if(ui-&gt;actionEasy-&gt;isChecked())
  {
      ui-&gt;actionEasy-&gt;setChecked(true);
  }
  </code></pre>
  
  <p>but this doent work and i dont understand why. </p>
  
  <p>As alternativ i could group radiobuttons, but i would like to unterstand why my code doent work.</p>