title: How to override some properties of TableView.rowDelegate while keeping others default
body: <p>I'm using a <code>rowDelegate</code> to implement variable row heights as follows.  However, it doesn't use the default background colors for selected/alternate rows. How can I preserve all defaults that I'm not trying to override?</p>
  
  <pre><code>TableView {
          id: messagesTable
          TableViewColumn {
              role: "severity"
              title: ""
              width: 20
              delegate: Image {
                  anchors.centerIn: parent
                  fillMode: Image.Pad
                  source: iconSources[styleData.value.toLowerCase()]
              }
          }
          TableViewColumn {
              role: "message"
              title: "Message"
              width: 300
          }
          TableViewColumn {
              role: "source"
              title: "File"
              width: 150
          }
          TableViewColumn {
              role: "startLine"
              title: "Line"
              width: 40
          }
          TableViewColumn {
              role: "startColumn"
              title: "Column"
              width: 50
          }
          rowDelegate: Rectangle {
              width: childrenRect.width
              height: (messagesModel.get(styleData.row).lineCount || 1) * 20
          }
          model: messagesModel
      }
  </code></pre>