title: PyQt5 set a QLabel x axis automatically center of the window
body: <p>How can I set <code>x</code> axis of the QLabel the exact center of the window? I have this example</p>
  
  <pre><code>from PyQt5.QtWidgets import QMainWindow,QApplication,QLabel
  from PyQt5 import QtCore
  import sys
  
  class cssden(QMainWindow):
      def __init__(self):
          super().__init__()
  
          self.mwidget = QMainWindow(self)
          self.setWindowFlags(QtCore.Qt.FramelessWindowHint)
          self.setFixedSize(700,400)
  
          self.label = QLabel(self)
          self.label.setText("Center of the window")
          self.label.setStyleSheet("color:green;"
                                         "font: bold 20pt 'Arial'")
          self.label.setGeometry(150,200,300,100)
          self.show()
  
  app = QApplication(sys.argv)
  app.setStyleSheet("QMainWindow{background-color: rgb(30,30,30);border: 2px solid rgb(20,20,20)}")
  ex = cssden()
  sys.exit(app.exec_())
  </code></pre>
  
  <p>What to do on <code>self.label.setGeometry</code> so the label will be on the center of the window all the time? Is there a method like <code>setCenter()</code> ?</p>