title: convert .ui to .py file which contains promoted frame
body: <p>I have a <a href="https://github.com/csete/gqrx/blob/master/src/qtgui/freqctrl.h" rel="nofollow noreferrer" title="freqctrl.h">.h file</a> and it's corresponding <a href="https://github.com/csete/gqrx/blob/master/src/qtgui/freqctrl.cpp" rel="nofollow noreferrer" title="freqctrl.cpp">.cpp file</a> which is a frequency controller widget.
  I can use it in QtCreator and c++ by promoting a Qframe widget to that .h file file without any problem.
  but I want to build an UI in QTDesigner and convert it to python using pyuic5.
  of course, pyuic5 cant not understand c++ codes and even there is no clue or link to the .cpp file in the .ui file! so how can I convert and use the form which contains the promoted QFrame to python code and use it in a pyqt application?</p>
  
  <p>here is my simple ui which I design and I want to convert to it to python using pyuic5</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;ui version="4.0"&gt;
   &lt;class&gt;MainWindow&lt;/class&gt;
   &lt;widget class="QMainWindow" name="MainWindow"&gt;
    &lt;property name="geometry"&gt;
     &lt;rect&gt;
      &lt;x&gt;0&lt;/x&gt;
      &lt;y&gt;0&lt;/y&gt;
      &lt;width&gt;456&lt;/width&gt;
      &lt;height&gt;280&lt;/height&gt;
     &lt;/rect&gt;
    &lt;/property&gt;
    &lt;property name="windowTitle"&gt;
     &lt;string&gt;MainWindow&lt;/string&gt;
    &lt;/property&gt;
    &lt;widget class="QWidget" name="centralwidget"&gt;
     &lt;widget class="CFreqCtrl" name="freqCtrl"&gt;
      &lt;property name="geometry"&gt;
       &lt;rect&gt;
        &lt;x&gt;80&lt;/x&gt;
        &lt;y&gt;70&lt;/y&gt;
        &lt;width&gt;280&lt;/width&gt;
        &lt;height&gt;40&lt;/height&gt;
       &lt;/rect&gt;
      &lt;/property&gt;
      &lt;property name="sizePolicy"&gt;
       &lt;sizepolicy hsizetype="Fixed" vsizetype="Fixed"&gt;
        &lt;horstretch&gt;0&lt;/horstretch&gt;
        &lt;verstretch&gt;0&lt;/verstretch&gt;
       &lt;/sizepolicy&gt;
      &lt;/property&gt;
      &lt;property name="minimumSize"&gt;
       &lt;size&gt;
        &lt;width&gt;280&lt;/width&gt;
        &lt;height&gt;40&lt;/height&gt;
       &lt;/size&gt;
      &lt;/property&gt;
      &lt;property name="toolTip"&gt;
       &lt;string/&gt;
      &lt;/property&gt;
      &lt;property name="frameShape"&gt;
       &lt;enum&gt;QFrame::StyledPanel&lt;/enum&gt;
      &lt;/property&gt;
      &lt;property name="frameShadow"&gt;
       &lt;enum&gt;QFrame::Raised&lt;/enum&gt;
      &lt;/property&gt;
     &lt;/widget&gt;
    &lt;/widget&gt;
   &lt;/widget&gt;
   &lt;customwidgets&gt;
    &lt;customwidget&gt;
     &lt;class&gt;CFreqCtrl&lt;/class&gt;
     &lt;extends&gt;QFrame&lt;/extends&gt;
     &lt;header&gt;freqctrl.h&lt;/header&gt;
     &lt;container&gt;1&lt;/container&gt;
    &lt;/customwidget&gt;
   &lt;/customwidgets&gt;
   &lt;resources/&gt;
   &lt;connections/&gt;
  &lt;/ui&gt;
  </code></pre>