Implement Thymio2 in target Enki playground

Currently the Thymio2 robot in Enki is an alias for the Epuck. For automatic testing it would be valuable to have a Thymio-II emulator in the Enki playground, that we could use both for validation and for long term testing.

In order to stress test movement and odometry thymio_motion.aesl and asebahttp/asebascratch we need at minimum:

• a motor event at 100 Hz, for the movement queue;
• native functions math.muldiv, math.cos, math.dot, math.min, math.clamp;
• sensors prox.horizontal and prox.ground; and
• motor variables motor.right.target and motor.left.target (which needed for the playground but not for odometry since they are read and written like ordinary variables)

To completely test thymio_motion.aesl as it is used for Scratch, we would need optionally the native functions for LEDs and sound

• leds.bottom.right, leds.bottom.right, leds.top
• sound.system, sound.freq

We don't really need the rest; the events and reporter variables for the other LEDs, sound recording, rc.tx/rc.rx etc will just not work.

A simple playground with a large flat table, black borders, and perhaps two straight black lines on side to test the calibration code would be useful for automatic testing.