Events like button, clap should be propagated on Aseba bus

[davidjsherman]

Currently asebascratch relies on Scratch's polling of state variables for informing it about events like button presses, but more intelligent clients could use event messages. Indeed ScratchX has some support for generating hat event messages.

These events are triggered by the robot's environment and are rare, so should not generate too much overhead. Thymio_motion.aesl could generate Aseba user events for them.

Candidates are: b_button/center, b_button/forward, b_button/backward, b_button/left, b_button/right, b_clap.