Finish chapter "Reuse and composition at work"

[koendehondt]

Open issues from https://github.com/SquareBracketAssociates/BuildingApplicationWithSpec2/pull/57:

• [ ] I think adding numbered labels to the different parts in the first figure would be helpful as a reference for the reader.
• [ ] The caption of Figure 8-5 reads: "Live coding your widgets (no it is not openWithSpec but open my friend but the background is supercool!". I understand the super coolness of the photo with the lighthouse in the background, but the code in the photo is wrong and the caption is not very professional. I think we should replace the figure and adapt the caption.
• [ ] In section 8.7 "Managing three widgets and their interactions", messages like add: #symbolHere are used, but I think that API had not been explained before.
• [ ] In section 8.10 "Changing the layout of a reused widget", the code just above "SHOULD Update" does not handle the case when there is no selection. Shouldn't we add that? Or does that complicate the example?
• [ ] Fix "SHOULD Update" and fix "ESTEBAN how can I do" in section 8.11 "Changing layouts".
• [ ] In section 8.11 "Changing layouts", there is a message "layoutAlternative1", but the method is not defined.
• [ ] In general, we should decide to use "widget" or "presenter". In this PR, I changed some "widget" to "presenter", but not all. Will we use them interchangeably, or is it better to use "presenter" all the way?

Other:

• [ ] Use screenshots in Pharo Light theme.