title: Updating gui items withing the process
body: <p>I am trying to make a GUI for my app and ran into a problem:
  using <code>PySimpleGUI</code> I have to define layout at first and only then display the whole window. Right now the code is like this:</p>
  
  <pre><code>import PySimpleGUI as sg      
  
  layout = [[sg.Text('Input:')],      
            [sg.Input(do_not_clear=False)],      
            [sg.Button('Read'), sg.Exit()],
            [sg.Text('Alternatives:')],
            [sg.Listbox(values=('value1', 'value2', 'value3'), size=(30, 2))]]      
  
  window = sg.Window('Alternative items', layout)      
  
  while True:      
      event, values = window.Read()      
      if event is None or event == 'Exit':      
          break      
      print(values[0])    
  
  window.Close()
  </code></pre>
  
  <p>Is it possible to only show the <code>Listbox</code> after the <code>Read</code> button is pushed? because I would only get values for <code>Listbox</code> after my input. Maybe it somehow possible to update the listbox with new values after button event?</p>