title: QLineedit disabling/re-enabling.
body: <p>I have an equation with four variables, only 3 need to be defined for the program to run. When 3 of the parameters have been defined (via lineedits in my GUI) I'd like the fourth to be greyed out. I only know how to link two parameters at the moment by disabling one lineedit when another is edited. e.g.</p>
  
  <pre><code>void WaveformGen::on_samples_textEdited(const QString &amp;arg1)
  {
      ui-&gt;cycles-&gt;setDisabled(true);
  }
  </code></pre>
  
  <p>So when samples is edited I disable another lineedit called cycles</p>