Improve system schedules

[ThierryBerger]

This builds on top of #576 to more easily visualize the impacts

• avoid systems in multiple sets:

This change doesn´t impact the execution, but orders systems more consistently, leading to simpler representation possible and less cognitive load.

Before

After

To be noted I'm not entirely sure keeping this RapierTransformPropagateSet is necessary, as a user might eventually want to order systems after systems, and there's no ambiguous beginning or end to this set.

Further improvements (out of scope)

Split RapierContext

There are systems running sequentially, but each write to different parts of RapierContext.

If we split RapierContext's:

• joints
• colliders
• bodies

We'd be able to parallelize some systems execution.

• I'm exploring this in a separate PR: Split RapierContext #585

[sebcrozet]

Thanks!