Support interactively sorting minibuffer completions

Support interactively sorting minibuffer completions

* lisp/minibuffer.el (minibuffer-completions-sort-function): New var.
(minibuffer-read-sort-order-with-completion)
(minibuffer-completions-sort-orders): New user options.
(minibuffer-sort-completions): New command.
(minibuffer-local-completion-map): Bind it to 'C-x C-v'.
(display-completion-list, minibuffer-completion-help): Take
'minibuffer-completions-sort-function' into account.
(completions-header-format): Add '%t' format spec construct,
substituted with a description of the current sort order.
* lisp/menu-bar.el (minibuffer-local-completion-map): Add menu bar
menu entry for sorting completions candidates.
* doc/emacs/mini.texi (Completion Commands): Document new command.
Improve documentation and indexing of 'minibuffer-complete-and-exit'.
(Completion Exit, Completion Options): Update
* doc/lispref/minibuf.texi (Completion Commands): Document new user
options and command.
* etc/NEWS: Announce new feature.