title: Qt - invalid use of incomplete type 'class QScrollBar' - Add horizontal scroll bar to text edit widget
body: <p>The default <code>QPlainTextEdit</code> has only vertical scroll bar, I want to add horizontal scroll bar.</p>
  
  <p>I tried this (this code in the constructor of the <code>QMainWindow</code> class)</p>
  
  <pre><code>QPlainTextEdit * editor = new QPlainTextEdit(this);
  QScrollBar * hScroll = new QScrollBar(Qt::Horizontal);
  editor-&gt;addScrollBarWidget(hScroll);
  setCentralWidget(editor);
  </code></pre>
  
  <p>but the build failed with the error (invalid use of incomplete type 'class QScrollBar').</p>