Finish chapter "A 10 min small example"

[koendehondt]

This PR fixes #72.

• All Pharo Dark images have been replaced with Pharo Light images
• The word "widget" has been replaced with "presenter" where possible.
• Text improvements
• Replaced the instance variable screen with result
• Use tabs for indenting code
• Adapted the code in the code repo.
• Ran all the code examples
• Removed old unused images and md files

[koendehondt]

@Ducasse I consider this chapter ready. Please review. If you agree with me, then this is the first finished chapter 😀.

[Ducasse]

Ok I will read now this one and the chapter 5 and we finish them.