Improvements to chapter 18 (A Simple Contact Book)

[koendehondt]

• Dropped Eg prefix of classes and packages, because the diagram under "Contact book model" does not include those prefixes.
• Added ContacnBook>>contents because it was missing but referenced under "Widget initialization".
• Replaced defaultSpec by defaultLayout.
• Adapted methods according to our guidelines (empty line after selector).
• Some text improvements here and there.

Open issue

• The diagram under "Contact book model" shows findContact, but the text includes ContactBook >> findContactsWithText:. The diagram has to be adapted.

[Ducasse]

Sound good.