Center ttk.Checkbox in grid column

[Rich Shepard]

On Sat, 8 Jun 2019, MRAB wrote:

> A workaround is to pack the widget in a tk.Frame and then put the frame in 
> the grid cell.

Okay. For now I'll leave it as-is and wait for feedback from users (if any)
when the application is up and running.

Thanks again,

Rich