group "link" and "focus" colors in theme layer

[Barsnes]

We currently only have "link" and "focus X" as colors exposed on the theme layer,
Maybe we should group these?

I suggest grouping them under "ds-extra", and not allowing a color with this name. We could invalidate all colornames with "ds-", if we forsee more things being added, but "extra" should cover most things.

[Barsnes]

We'll leave it as it is now, but see if we need this if we get one or more things.