This example shows the checkbutton widget.

A checkbutton widget is a on/off switch. The state of the switch is true when it is on, else it is false.

- The init parameter sets the initial state of the switch.
- The handle parameter allows to specify a variable that can change the state of the switch at any time.
- The return parameter specify a variable that will be binded to the value (true or false) of the checkbutton when the window is closed.
- The action parameter specify a command to execute just after the switch has changed its state because of a user input.
