New commands for making icomplete behave more like ido

New commands for making icomplete behave more like ido

This tries (not particularly hard) to emulate Ido's magic C-k, C-d,
RET and DEL.  For now, the new commands have to be explicitly added to
an active map by the user, using something like this:

(let ((imap icomplete-minibuffer-map))
  (define-key imap (kbd "C-k") 'icomplete-magic-ido-kill)
  (define-key imap (kbd "C-d") 'icomplete-magic-ido-delete-char)
  (define-key imap (kbd "RET") 'icomplete-magic-ido-ret)
  (define-key imap (kbd "DEL") 'icomplete-magic-ido-backward-updir))

* lisp/icomplete.el (icomplete-magic-ido-kill)
(icomplete-magic-ido-delete-char, icomplete-magic-ido-ret)
(icomplete-magic-ido-backward-updir): New commands.