Translating images and cheatsheet into Dutch

This pull request contains a Dutch translation of the images in the lesson_files. I intend to also translate the code snippits of the respective lessons sometime soon, but I figured I might as well submit the images right now. I also translated the cheatsheet, as that appeared not to break any related references from the lesson material anyway.

I was unable to run `mogrify` successfully to obtain the same PNGs from the PDFs that you produce, as I'm probably on a different version of ImageMagick (it produced a weird alpha-channel bug). I hope it's not too much inconvenience for you to render the PNGs from the PDFs.