Refactor structure and naming of skin-related classes

[peppy]

This is a full pass on the skin namespace, focused on renaming, documenting and generally knocking some (final?) sense into the structure and understandability before I attempt to make things more complicated.

• Depends on Simplify and rename SkinnableTargetComponentsContainer #22647 (conflict avoidance)

Note that I've intentionally avoided changing any naming or namespaces which would break existing usages / serialisation.

[bdach]

Difficult diff to get through but that was to be expected. Generally feel that this is a net gain in legibility. Left some minor comments, nothing too major - whether they're actionable is mostly up to you.

[bdach]

I think I'm more or less fine with this now, save for the one remaining review thread - but just so I know for the near future (and can maybe expedite this a bit faster without having to go through the usual call-and-response), does this review request mean that you want an approval from @frenzibyte specifically here before merging?

[peppy]

does this review request mean that you want an approval from @frenzibyte specifically here before merging?

I was hoping for one, but I don't think it should hold things up so feel free to bypass it. This is going to be a dependency for all my forward work, so I'd prefer it merged ASAP so I can continue moving forward unencumbered.

[frenzibyte]

Reads pretty well to me. Nice one 👍🏻. @bdach requesting a final review from you since you've reviewed this before.