I attached a patch which adds grouping support to the completions 
buffer. The completion table can specify a `group-function` via its 
completion metadata.

Currently groups are only displayed in the *Completions* buffer if 
`completions-format=one-column`. My proposal is to split up the function 
`completion--insert-strings` into three functions 
`completion--insert-strings-one-column/vertical/horizontal` in a 
follow-up patch. Then the grouping display can also be implemented for 
the other display formats.

Group title support is already present in the Vertico, Selectrum and 
external Icomplete-vertical UI. Similarly we can add support to the 
recently added `icomplete-vertical-mode` by Gregory.

There is a patch to `xref.el` included, which makes use of the grouping 
functionality, when the `completing-read` show function is used:

(setq xref-show-xrefs-function #'xref-show-definitions-completing-read)

Daniel