bug#68214: Completion sorting customization by category

[Juri Linkov]

>> This is what I already considered but hesitated to make this change
>> since it modifies the default behavior.  OTOH, probably there is
>> no possible harm from this change.  So I'll give this a try.
>
> Thanks. Seems better to take that route, instead of changing all
> `completion-metadata-get' calls to `completion-metadata-override-get'
> and leaving `completion-metadata-get' mostly unused. As long as we don't
> modify `completion-category-defaults' or `completion-category-overrides'
> there won't be a change in behavior.

Thanks for the suggestion.  Now pushed in commit 18de131222e,
so all metadata properties are supported now in
`completion-category-overrides'.

What do you think about doing the same for
`completion-extra-properties'?

This basically means moving `plist-get completion-extra-properties'
to `completion-metadata-get' that will replace this:

  (ann-fun (or (completion-metadata-get all-md 'annotation-function)
               (plist-get completion-extra-properties
                          :annotation-function)))
  (aff-fun (or (completion-metadata-get all-md 'affixation-function)
               (plist-get completion-extra-properties
                          :affixation-function)))

with just:

  (ann-fun (completion-metadata-get all-md 'annotation-function))
  (aff-fun (completion-metadata-get all-md 'affixation-function))