title: How to implement a TreeViewer such that root elements have multiple cells but child elements only a single cell?
body: <p>The following code shows how to create a <code>TreeViewer</code> with columns.</p>
  
  <pre><code>TreeViewer treeViewer = new TreeViewer(shell, SWT.BORDER);
  Tree tree = treeViewer.getTree();
  tree.setHeaderVisible(true);
  tree.setLayoutData(new GridData(SWT.FILL, SWT.FILL, true, true, 1, 1));
  
  TreeViewerColumn treeViewerColumn = new TreeViewerColumn(treeViewer, SWT.NONE);
  TreeColumn trclmnNewColumn = treeViewerColumn.getColumn();
  trclmnNewColumn.setWidth(100);
  trclmnNewColumn.setText("New Column");
  
  TreeViewerColumn treeViewerColumn_1 = new TreeViewerColumn(treeViewer, SWT.NONE);
  TreeColumn trclmnNewColumn_1 = treeViewerColumn_1.getColumn();
  trclmnNewColumn_1.setWidth(100);
  trclmnNewColumn_1.setText("New Column");
  </code></pre>
  
  <p>By default, if you expand a root element row, child element rows appear, and just like the root element rows, they are also divided into cells by the columns.</p>
  
  <p>I would like to implement a <code>TreeViewer</code> such that child elements, unlike root elements, consist of a single cell that spans the width of the <code>TreeViewer</code>.</p>
  
  <p>How can this be done?</p>