title: How to fully close tkinter-app?
body: <p>Following code is given:</p>
  
  <pre><code>import tkinter as tk
  
  
  class Application(tk.Frame):
      def __init__(self, master=None):
          tk.Frame.__init__(self, master)
          self.grid()
          self.createWidgets()
  
      def createWidgets(self):
          self.quitButton = tk.Button(self, text='Quit', command=self.destroy)
          self.quitButton.grid()
  
  
  app = Application()
  app.master.title('Sample application')
  app.mainloop()
  </code></pre>
  
  <p>When running this code a small window appears with a QUIT-Button in it. The button disappears when pressed but the window stays. 
  How to close the window and the app with that Button-click?</p>