Refine grouping based around W3C CSS specification modules

[stormwarning]

New CSS properties are arriving thick and fast, and it's getting difficult to determine which group a given property should belong in. Some of the existing groups like "Typography" and "Interactions" are also getting a bit unwieldy.

I'm going to start using the W3C's specification modules as a way to help determine the grouping when it's not clear from the existing properties. This should be possible without much difference to the existing property order, but some properties will end up in a different order than they currently do. This will be marked as a breaking change of course.