title: Filtering in QFileDialog
body: <p>I would like to filter the files that are shown in a <code>QFileDialog</code> more specifically than just by file extensions. The examples I found in the Qt documentation only show filters like <code>Images (*.png *.xpm *.jpg);;Text files (*.txt);;XML files (*.xml)</code> and such. In addition to this I would also like to specify a filter for files that should <em>not</em> show up in the file dialog, e.g. <code>XML files (*.xml)</code> but not <code>Backup XML files (*.backup.xml)</code>.</p>
  
  <p>So the problem I have is that I would like to show some files in the file dialog that have certain file extension, but I would not like to show other files with a specific file name suffix (and the same file extension).</p>
  
  <p>For example:</p>
  
  <p>Files to show:  </p>
  
  <pre><code>file1.xml  
  file2.xml
  </code></pre>
  
  <p>Files not to show:  </p>
  
  <pre><code>file1.backup.xml  
  file2.backup.xml
  </code></pre>
  
  <p>I would like to ask if it is possible to define filters like these for a <code>QFileDialog</code>?</p>