Tidy some things up in modules/jolt_physics

[mihe]

I'm sure I could find many more things to change, but I've had some of these lying around in a branch for months now, waiting for more of these minor changes to pile up, and now seems like a good time to push them.

The changes are as follows:

1. Moved JoltObject3D::shapes into JoltShapedObject3D instead, since it had no business being there, so just added 16 bytes to every JoltSoftBody3D for no reason.
2. Have the creation of JPH::EmptyShape just use the constructor rather than going the roundabout way of JPH::EmptyShapeSettings(...).Create().Get().
3. Moved the recently moved SetBodyActivationListener calls from JoltSpace3D::step into JoltSpace3D::_pre_step and JoltSpace3D::_post_step, where they arguably belong.
4. Changed JoltContactListener3D::area_soft_body_overlaps to be a LocalVector instead of a HashSet, since we have no need for the deduplication, sorting or lookup that HashSet would offer.

[Repiteo]

Thanks!