[EPIC] Improve the playground

The current code of the playground is a bit more than a hack which I wrote in a hurry.

I think there is a lot of room for improvement, so I'm opening this issue as a starting point for anyone who wants to contribute to it. If anyone has an idea for an improvement and/or wants to work on a particular aspect, feel free to open a sub-issue.

Here is a random list of ideas to improve it in various ways:

1. UI improvements: the current look is not particularly good. It would be nice e.g. to have the editor/terminal automatically resized to fit the window and/or being able to decide on vertical vs horizontal split and/or being able to resize them. They playground is using LTK but it's not a requirement, I'd be happy to use a different framework (or no framework at all) if someone has opinions.

2. #311

3. #302

4. #308

5. #307

6. #312

7. Improve the examples selection. Would be nice to have more examples, possibly ordered from least to most complex, and show the various spy capabilities

8. anything else you can think of, happy to discuss :)