title: QWidget.setContentsMargins() appears to be ineffective: why so?
body: <p><strong>Situation:</strong><br>
  I am working on a Qt4 application constructed in this way (in case parent widgets matter in this issue):</p>
  
  <pre><code>QApplication
     |_ QMainwindow
           |_ QScrollArea (central widget)
                 |_ QFrame (child of scroll area)
                       |_ QFrame
                       |      |_ QLabel
                       |            |_ QPixmap
                       |_ QFrame
                       |      |_ QLabel
                       |            |_ QPixmap
                       |_ QFrame
                             |_ ect...
  </code></pre>
  
  <p><strong>Objective:</strong><br>
  I want there to be no margins between the sub-QFrames and their QLabels and equally between QLabels and their QPixmap.</p>
  
  <p><strong>Method:</strong><br>
  I have requested to reduce the sub-QFrame’s margins with QFrame.setContentsMargins(0, 0, 0, 0) and with its layout’s QBoxLayout.setSpacing(0). Zero-margin between QLabel and its QPixmap seems to occur naturally.</p>
  
  <p><strong>Problem:</strong><br>
  In spite of all this, margins within QFrames persist in showing up: a situation which I have been able to test by applying a Style Sheet to the various widgets.</p>
  
  <p>What can I do?</p>