[Refactor] Switch to FreeSimpleGUI

[FreeSimpleGUI](https://github.com/spyoungtech/freesimplegui) is a continuation of PySimpleGUI beginning from the last open source release. It would be helpful to test compatibility of your GUI builder with this newer package.

Testing is easy, and merely requires changing the import:

```diff
- import PySimpleGUI as sg
+ import FreeSimpleGUI as sg
```