Rework our class structure and naming to match the official specs

[oliverklee]

https://www.w3.org/TR/cssom-1/
https://www.w3.org/TR/css-syntax-3/

We'll probably need to use PHP class aliases and deprecate a lot of things.

[JakeQZ]

I'm not so sure.

The referenced documents are not an object model, but a syntax.

Current class hierarchy seems to fit. At least for me. But, yes, please review.

[JakeQZ]

Actually, I think there are some misnomers. E.g. the current Rule\Rule class should be a Declaration or CssDeclaration in some namespace (ref). And the Rule and Property namespaces seem to be transposed from their current official definitions: a 'rule' is an enclosing block; a 'property' is a single value.