Provide a single-notebook app

[nthiery]

Having dedicated notebooks for each level is nice to provide strong guidance to beginners, and to help them keep a record of their solutions to each level.

Nevertheless, it would be nice to provide a single app-style notebook letting the user switch swiftly between levels and languages (as a step toward #5). This would mimic more closely the original laby application.

Such a notebook could be generated automatically from the multilingual descriptions of the levels from the original laby application, as provided in the level metadata. app-mode or RISE could presumably be used to show just the relevant info, including the code cell, and automatically launch laby (at this stage, can we actually achieve both?).

Bonus: support multiple programming languages #6. Just by changing the kernel? How to provide the initial language-specific code stub?