[css-ui][editorial] Markup <x> and <y> productions with <dfn> element

[cdoublev]

I could not find the issues mentioned in the message of 9b8f394 in the archives, so I may be missing the reason not to do this change. But it would allow w3c/reffy to extract these CSS types.

[tabatkins]

I would much rather just lightly rewrite the spec to use <<number>> in the grammars and define that they're the X and Y coordinates in prose. The custom production doesn't add anything here, and differs from how we do similar grammars in many other specs.

[tabatkins]

All right, editorial fixup of UI 3 and 4 committed, so I'm closing this.