Playground walls can have angles and mass, plus an example maze

Wall elements in playgrounds are defined by a width, length, and height around a center. This PR adds an ‘angle’ attribute to wall elements in order to specify a rotation around the center, expressed in radians.

As an illustration, examples/thymio-maze-6x6.playground contains a 6x6-cell maze with two Thymios, that can be used to test navigation strategies based on wall avoidance or on track following.

Here is a sample run with one Investigator robot and one Explorer robot: