title: PyQt4 : check the window is existing or not
body: <p>I made a kind of tool for MAYA.
  Once I call the class and make a instance, I don't have to call it anymore.
  Instead I must check the window is existing or not.
  In fact, when I press a button to call <code>close()</code> or "X" button, it doesn't call <code>__del()__</code> method. I can't clear up my works.</p>
  
  <p>So, I plan to check the instance is existing, and if it is, I don't call class, just call <code>show()</code>.
  But, I can't find the way.</p>
  
  <pre><code>_win = RigControlWindow()
  _win.show()
  </code></pre>
  
  <p>How can <code>RigControlWindow</code> class find the instance is existing?</p>