css.lang: Rename selector style/context IDs
css.lang: Rename selector style/context IDs

This renames the IDs of selector styles and contexts to follow the
naming conventions used for all other styles/contexts. This also splits
the selector-grammar context, to label the symbols using their actual
(CSS spec) names.

https://bugzilla.gnome.org/show_bug.cgi?id=796130

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