title: add a layout to another layout from the parent window
body: <p>I am new to this field, and I Hope this not seems a stupid question
  I am using PyQt5 for Python 3.8 on mac
  I want to add a layout(addLayout) to another layout from the parent window if it is possible!</p>
  
  <p>This is an example: I have a Class (ParentWindow) it has many widgets and hboxlayout. (ChildWindow) is a class inherited from ParentWindow and it has also widgets and other layout; the question: can I add layout in the child window?
  If I use setLayout in the ChildWindow it ignores it and show the message:
  (QWidget::setLayout: Attempting to set QLayout "" on ChildWindow "", which already has a layout) So, can I use addLayout to the parent window layout? And how </p>
  
  <pre><code># The Parent Class
  from PyQt5.QtWidgets import  QWidget, QHBoxLayout,QLabel
  class ParentWindow(QWidget):
      def __init__(self):
          super().__init__()
          title = "Main Window"
          self.setWindowTitle(title)
          left = 000; top = 500; width = 600; hight = 300
          self.setGeometry(left, top, width, hight)
          MainLayoutHbox = QHBoxLayout()
          header1 = QLabel("Header 1")
          header2 = QLabel("Header 2")
          header3 = QLabel("Header 3")
          MainLayoutHbox.addWidget(header1)
          MainLayoutHbox.addWidget(header2)
          MainLayoutHbox.addWidget(header3)
          self.setLayout(MainLayoutHbox)
  # End of Parent Class
  
  
  # The SubClass
  
  from PyQt5.QtWidgets import QApplication,   QVBoxLayout, QHBoxLayout,  QTabWidget, QLabel
  import sys
  
  from ParentWindow import ParentWindow
  
  
  class ChildWindow(ParentWindow):
      def __init__(self):
          super().__init__()
          vbox = QVBoxLayout()
          MainLabel= QLabel("My Window")
  
          vbox.addWidget(MainLabel)
  
          self.setLayout(vbox) # This show the Warning  Error
  
          # I assume the below code should work to extend the parent layout!! But it gives error
          # parent.MainLayoutHbox.addLayout(vbox)
  
  
  
  
  if __name__ == "__main__":
      App = QApplication(sys.argv)
      MyWindow= ChildWindow()
      MyWindow.show()
      App.exec()
  </code></pre>