Lesson 16

 No real surprises in this lesson; we are learning how to open

a new window from main. Three scenarios are examined: a toplevel

window, on top of the calling window. Here, closing the bottom window

closes the top one as well but not vice versa. One can just close the top one. 

Toplevel() is a built-in feature.

                                                                

The second case is just two independent windows: one can just close the one

or the other.

                                                                           

In the third situation, opening a new window destroys the one already openend thanks to

a destroy() function.

                                                                             

Clean!!