(minibuffer-completion-help): Add group sorting

(minibuffer-completion-help): Add group sorting

Sort the groups as returned by the `group-function` of the completion
table depending on the value of the customizable variable
`completions-group-sort`.  By default `completions-group-sort` is set
to nil.  The variable can be set to the symbol `alphabetical` in order
to configure alphabetical sorting.  Furthermore, a custom sorting
function can be used as value of `completions-group-sort`.

* lisp/minibuffer.el (completions-group-sort): New variable.
(minibuffer--group-by): Add SORT-FUN argument.
(minibuffer-completion-help): Pass `completions-group-sort` to
`minibuffer--group-by`.