title: PyQt: QFileDialog.getExistingDirectory using a default directory, user independant
body: <p>When using the <code>QFileDialog.getExistingDirectory</code> I've found the way to specify the default path to a directory. I wanted to use a default directory somewhere in my (user) home folder on my Linux (Mint) machine. I used:</p>
  
  <pre><code>my_dir = QtGui.QFileDialog.getExistingDirectory(
      self,
      "Open a folder",
      "/home/my_user_name/",
      QtGui.QFileDialog.ShowDirsOnly
      )
  </code></pre>
  
  <p>Which worked beautifully. The next 'level' is now to be able to do this for every user. I tried the standard Linux way and change <code>/home/my_user_name/</code> to <code>~/</code>. That did not work. It resulted in the working directory instead of the user's dir. </p>
  
  <p>Should I use more system arguments when calling my app from the terminal? Or am I missing a PyQt function? </p>