Finish chapter "Most of Spec in one example"

[koendehondt]

This is the PR for #73.

It was quite some work to finish this long chapter. 😅

• The code has been changed accordingly. See Organize all classes in one package "CodeOfSpec20Book" with a tag per book chapter CodeOfSpec20Book#4.
• A missing section to introduce the modal dialog has been added.
• All images have been replaced. They look much better now. They include correct film data.
• Many improvements to the text.
• See the checklist in Finish chapter "Most of Spec in one example" #73 for more stuff that has been done.

[koendehondt]

@Ducasse If you do not have any comments or suggestions, I consider the chapter "Most of Spec in one example" to be finished. One more done! 😎

[Ducasse]

I will have a look and indeed it is long and it cost me time also in the past.

[Ducasse]

I read the complete chapter in speed mode and it looks good to me.

[koendehondt]

@Ducasse I overlooked that a test did not pass anymore after changing #setModel: to cope with a nil argument. I adapted the test and changed it in the book and in the code repo. I pushed the changes to the two master branches.

[Ducasse]

Cool!
I'm massively cleaning my house after the facade renewal (dust everywhere).