[Plan] Gradually replace passing around RenderArrays with RenderableInterface objects

Problem/Motivation

In an age where static analysis thrives, passing around RenderebleInterface is preferrable over passing "naked" RenderArrays. (An array can be anything, a Renderable is typed.)

Proposed resolution

Gradually replace passing around render arrays w/ passing around RenderableInterface objects (in child issues).

Some thoughts:
- Provide a blessed Renderable object with good DX in core.
- Allow RenderableInterface as a return value e.g. in EntityInterface::label(). (How can a deprecation dance be done for a return type extension? Prior examples?)

Remaining tasks

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