Add difficulty meter to worlds

[aroberge]

For students looking at worlds randomly, having a difficulty meter might be useful. For example, while world Rain 1 might be relatively easy, Rain 2, which looks very similar, is comparatively extremely difficult for beginners, due to the logic involved in identifying an open window.

• [x] implementation
• [ ] documentation

[aroberge]

To try it, load World("/worlds/levels.json") and look at the world's info.

[aroberge]

Moved to Reeborg-howto