Finish chapter "Reuse and composition at work"

[koendehondt]

Open issues from #57:

• I think adding numbered labels to the different parts in the first figure would be helpful as a reference for the reader. Decided not to do this.
• 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? Decided not to do this.
• 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? Decided not to do this.

Other:

• Use screenshots in Pharo Light theme.