title: QML ListModel onDataChanged issues
body: <p>I'm trying to implement a callback function on my QML ListModel to do some operations when the data inside the ListModel changes. Apparently, it's impossible to create on...Changed handlers for individual user-defined properties in a ListModel. So, I'm trying to use the onDataChanged handler available to ListModel.</p>
  
  <pre><code>ListModel {
      id: model
  
      onDataChanged: {
          console.warn("onDataChanged: "+get(topLeft).val + " vs. " + get(bottomRight).val)
      }
  }
  </code></pre>
  
  <p>However, onDataChanged only ever seems to reference the very first element in the ListModel. That is to say, topLeft and bottomRight always point to the first element, even when I manipulate an element down the list.</p>
  
  <p>What I need here is a reliable handler that accurately reflects the correct row in the ListModel when it fires off.</p>