PR to discuss/merge Inria's contributions to Enki

Hello Thibault,

Thank you very much for this cleaning work. Here are some general questions:

I do not understand this step: Move frame rate syncronisation from Viewer class to EnkiPlayground class, as this would break the compilation of Aseba programs using Enki. Can you explain why you did so?
In commit 416580aa8b7f6e6d0fde5261e618e37e44da8874 you add support to hook things into the viewer from outside a subclass. Why this design choice?
In commit bf7f5b5ee910bd8be7e3fde470ae7da7453345aa you implement a way to save a scene, but it is stored inside the viewer. I would like to discuss this feature in more details, maybe it could be generalised. The current version gives me the feeling that it is a bit hacky.
The awesome Warhammer themed texture might be a violation of copyright. Moreover it is not really what we aim at with the Thymio. This texture should not be in the history (for copyright reasons), but we should give a way for the users of Enki to easily overlay some decals on the base texture :)
I would like to discuss the LED texturing by videoconferencing.
Other comments are discussed in the specific commits.

enki-community / enki