Finish chapter "Commander: A powerful and simple command framework"

[koendehondt]

• [ ] Check all code examples and images

Open issues from https://github.com/SquareBracketAssociates/BuildingApplicationWithSpec2/pull/63:

• [ ] testRemoveContact under "Adding the Remove Contact command" is not a good test. The initial items are not specified and more importantly, the test does not verify that the selected contact has been removed. It just checks that there is one contact less than initially.
• [ ] Under "Enabling shortcuts", it reads "At the time of writing this chapter, Commander management of shortcuts has not been pushed to Spec to avoid dependency on Commander.". Is that still correct? I did not check that yet.
• [ ] This chapter mentions toolbars, but it does not add a toolbar to a window.