Consider introducing System as a concept to fully align with ECS model.

[jleibs]

At the moment our Scenes act in a "System-like" way, but without the formalism or framework support that one might find in a true ECS.

Given we are already using the terminology Entity and Component and doing System-like things it could be beneficial to go the rest of the way here. Systems could be a nice way to reconcile some other ideas we have discussed such as:

• Application of annotations
• Derived component values
• Data filtering / highlighting

[Wumpf]

We introduced Systems by now in the form of ViewPartSystem and ContextSystem. In the future we'll formalize this more, but System already started to become a building block in our architecture.

Se also https://github.com/rerun-io/rerun/blob/main/design/space_views.md