title: Update a checkbox based on selected values of another checkbox
body: <p>I want to have a select all checkbox in my PySimpleGUI. When the select all checkbox is selected, all other checkboxes should change to True, and if any other box is unchecked, the select all checkbox should change to false state?</p>
  <p>I can do it through button clicks, but I couldn't find a way to update checkboxes based on values selected in an another checkbox?</p>
  <pre><code>import PySimpleGUI as sg
  layout = [
      [sg.Checkbox ('select all', key = 'checkbox')],
      [sg.Checkbox ('value 1', key='check_value1')],
      [sg.Checkbox ('value 2',key='check_value2')],
      [sg.Button ('ON', key = 'ON')],
      [sg.Button ('OFF', key = 'OFF')]
  ]
  window = sg.Window ('Sample GUI', layout) .Finalize ()
  while True: # Event Loop
      event,values=window.read()
      if event in (None, 'Exit'):
          break
      elif event == 'ON':
          window ['checkbox']. Update (value = True)
      elif event == 'OFF':
          window ['checkbox']. Update (value = False)
      print(event,values)
  window.close ()
  </code></pre>
  <p>Is there any way to implement this?</p>