Rework broken icomplete-buffer into icomplete-in-buffer-mode

Rework broken icomplete-buffer into icomplete-in-buffer-mode

* etc/NEWS (Icomplete): Mention icomplete-in-buffer-mode.

* lisp/icomplete.el (icomplete-in-buffer): Make obsolete.
(icomplete-mode): Use icomplete-in-buffer-mode.
(icomplete--field-string): Use completion-in-region--data.
(icomplete-minibuffer-setup): Don't setup completion-in-region here.
(icomplete-exhibit): Call icomplete--matches-summary.
(icomplete--matches-summary): Hew helper.
(icomplete-in-buffer-mode): New minor mode.
(icomplete-in-region, icomplete-in-region-complete)
(icomplete--saved-vars, icomplete--setup-vars)
(icomplete--restore-vars, icomplete--in-region-setup): New
functions.