title: How to add new label in window?
body: <p>I have a window. There is a button. When the user clicks on this button, in the window must disappear button and appear new label and new button. 
  Do you understand?</p>
  
  <pre><code># -*- coding: utf-8 -*-
  
  from PyQt4 import QtGui, QtCore
  import sys
  def starting():
      quest1 = QtGui.QWidget()
      quest1.setWindowTitle('New')
      quest1.resize(900, 600)
      quest1.show()
      quest1.exec()
  
  
  testing = QtGui.QApplication(sys.argv)
  
  window = QtGui.QWidget()
  window.setWindowTitle('Title')
  window.resize(900, 600)
  
  MainText = QtGui.QLabel('&lt;p align="center"; style="font-size: xx-large"&gt;Text&lt;/p&gt;')
  Mainbox = QtGui.QVBoxLayout()
  
  # buttons
  start = QtGui.QPushButton('Start')
  quit = QtGui.QPushButton('Exit')
  start.setFixedSize(70, 40)
  quit.setFixedSize(70, 40)
  
  buttons = QtGui.QHBoxLayout()
  buttons.addWidget(start)
  buttons.addWidget(quit)
  # /buttons
  
  
  Mainbox.addWidget(MainText)
  Mainbox.addLayout(buttons)
  
  window.setLayout(Mainbox)
  
  QtCore.QObject.connect(quit, QtCore.SIGNAL('clicked()'), QtGui.qApp, QtCore.SLOT("quit()"))
  QtCore.QObject.connect(start, QtCore.SIGNAL('clicked()'), starting)
  
  window.show()
  sys.exit(testing.exec_())
  </code></pre>
  
  <p>I think, I do wrong, is not required to make new window, but I don`t know what I must doing.</p>