Initial Checkin of new callback demo

DemoPrograms/Demo_Button_Func_Calls.py

@@ -0,0 +1,43 @@
+#!/usr/bin/env python
+import sys
+if sys.version_info[0] >= 3:
+    import PySimpleGUI as sg
+else:
+    import PySimpleGUI27 as sg
+
+
+"""
+Demo Button Function Calls
+Typically GUI packages in Python (tkinter, Qt, WxPython, etc) will call a user's function
+when a button is clicked.  This "Callback" model versus "Message Passing" model is a fundamental
+difference between PySimpleGUI and all other GUI.
+
+There are NO BUTTON CALLBACKS in the PySimpleGUI Architecture
+
+It is quite easy to simulate these callbacks however.  The way to do this is to add the calls
+to your Event Loop
+"""
+
+def callback_function1():
+    sg.Popup('In Callback Function 1')
+    print('In the callback function 1')
+
+def callback_function2():
+    sg.Popup('In Callback Function 2')
+    print('In the callback function 2')
+
+layout = [ [sg.Text('Demo of Button Callbacks')],
+           [sg.Button('Button 1'), sg.Button('Button 2')] ]
+
+window = sg.Window('Button Callback Simulation').Layout(layout)
+
+while True:             # Event Loop
+    event, values = window.Read()
+    if event is None:
+        break
+    elif event == 'Button 1':
+        callback_function1()        # call the "Callback" function
+    elif event == 'Button 2':
+        callback_function2()        # call the "Callback" function
+
+window.Close()