bug#5139: 23.1.50; feature request: completion-buffer-sort-function

When a *Completions* buffer is generated by minibuffer-completion-help
it will sort the possible completions using string-lessp. It would be
nice if the sorting order was not hard-coded but could be modified
according to context. That's why I want to suggest to replace the
call of string-lessp by something like

 (funcall completion-buffer-sort-function)

so that completion-buffer-sort-function can be bound by the caller
to something else according to context.

Kind of related:

Such a function could do more than sorting the completions (or being
a sort predicate). It could also help formatting the *Completions*
buffer. Say, a *Completions* buffer can be hard to read when it
contains many entries. If it lists, for example, buffer names, it
could sort the buffer names by mode and put some kind of seperator
between groups of buffers with the same mode. Then this seperator
could be translated into something like an empty line in the
*Completions* buffer.

Roland