title: QML Row: How to extend qml containers (Row or Column) from other file
body: <p>It seems should have a solution for sure.
  Suppose I have a Test.qml file containing this:</p>
  <pre><code>import QtQuick 2.0
  
  Rectangle {
      color: &quot;green&quot;
  
      Row {
          id: row
          spacing: 10
          anchors.fill: parent
          Rectangle {
              color: &quot;red&quot;;
              width: 100;
              height: 100;
          }
          Rectangle {
              color: &quot;red&quot;;
              width: 100;
              height: 100;
          }
          Rectangle {
              color: &quot;red&quot;;
              width: 100;
              height: 100;
          }
      }
  }
  </code></pre>
  <p>Now suppose we want to use this Test.qml within another file like main.qml:</p>
  <pre><code>import QtQuick 2.15
  import QtQuick.Window 2.15
  
  Window {
      id: window
      width: 640
      height: 480
      visible: true
      title: qsTr(&quot;Hello World&quot;)
  
  
      Test {
          anchors.fill: parent;
          // I want to be able to add new items (rects) to the row inside Test.qml
      }
  }
  </code></pre>
  <p>Now suppose we want to extend items to the row object in Test.qml, But we want to add from main.qml. How we can do that? is that even possible?</p>
  <p>(FYI: The application of this feature would be to develop a placeholder form and fill the items in the other items so we can skip duplicate codes. )</p>