title: QML uncheck checked button in ExclusiveGroup
body: <p>I have three buttons that are in an <code>ExclusiveGroup</code>:</p>
  
  <pre><code>ExclusiveGroup {id: group}
  Button{
      checkable: true
      exclusiveGroup: group
  }
  Button{
      checkable: true
      exclusiveGroup: group
  }
  Button{
      checkable: true
      exclusiveGroup: group
  }
  </code></pre>
  
  <p>They are obviously all unchecked before I click any of them, but how can I uncheck them once one of them are checked? Do I really need to add another button that, once checked, produces the behaviour that would apply when none of the other buttons are checked?</p>