bug#68214: Completion sorting customization by category

[Juri Linkov]

> Now I tried to use a more user-friendly let-binding of
> 'completion-extra-properties' like below, but then discovered
> that it doesn't work for the category.
>
> Maybe 'completion-metadata-get' could try to get a category
> from 'completion-extra-properties' as well?

Sorry, this was imprecise.  I meant another layer of indirection:
to get a category from 'completion-extra-properties'
to be able to get 'display-sort-function' by category.