Implement comments from #428

[hageboeck]

This is the implementation of many remarks / suggestions made in #428, such as:

• Better explain header/source separation
• Add some code highlights or drawings
• Discuss object slicing
• Add possible operator symbols for overloading
• Move this slide to a later stage of the OO chapter (and change the confusing code example)

I decided to disregard many comments from #428, because there was no reasonable way to change the slides. These comments referred rather to what we say or questions that the students asked. All the things I did implement are marked with tick boxes, and the things I left out have empty boxes in #428. I don't see a way to do better, so I propose to close the issue after this PR.

Resolves #428

[sponce]

@hageboeck can you rebase so that pdf can be built and we can see the result ?

[hageboeck]

can you rebase so that pdf can be built and we can see the result ?

@sponce The CI completed now.

[hageboeck]

The parts where we couldn't agree are removed now. I think that should be enough now to close #428.