Change default coefficient of restitution to 0.0

[Jondolf]

Currently, the coefficient of restitution is 0.3 by default. This is quite arbitrary, and can lead to confusion when specifying a restitution of zero for only one entity, as there will still be some bounciness, because collisions (by default) use the average of the coefficients of the colliding entities.

The default should be changed to 0.0. This is consistent with all other game engines and physics engines I've checked, such as Godot, Unity, Box2D, and Rapier.

Note that it might make sense to change the default friction coefficient as well. Currently, it is 0.3, but most engines use either 0.6 or 1.0. This may be worth its own issue though.