Merge Challenge and Marxbot simulators into Playground

Created by: stephanemagnenat

It makes no sense to have three simulator applications, all using Enki, to be shipped with Aseba. Playground is the most generic framework. Challenge and Marxbot should be merged into it and shipped as a single application, Aseba Simulator.

The requirements are:

For Marxbot:

• modification of incoming data path into Playground's model
• move of the Marxbot code into Playground

For Challenge:

• modularisation of Challenge's score system into Playground's architecture
• adding of scene-dependent help system to Playground
• adding of a "simple e-puck robot" to Playground from Challenge
• recreation of Challenge's scenario as Playground scene