Major rewrite to behave more like a minor mode.
Doc fixes.
(icomplete-mode): New function and var.
(icomplete-minibuffer-setup-hook): New hook.
(icomplete-inhibit): Defunct. Using icomplete-pre-command-hook,
icomplete-post-command-hook as real hook functions.
(icomplete-simple-completing-p): Subsumes
icomplete-window-minibuffer-p plus other stuff, determines whether
simple completion is active.
(icomplete-window-minibuffer-p): Defunct, see above.
(icomplete-minibuffer-setup): Use icomplete-simple-completing-p,
copy-sequence hooks before adding to them, run hooks properly.
(icomplete-tidy): Changed from misnamed icomplete-pre-command-hook;
use icomplete-simple-completing-p.
(icomplete-exhibit): Changed from misnamed icomplete-post-command-hook;
use icomplete-simple-completing-p.
(icomplete-completions): Changed from old, misnamed icomplete-exhibit;
combined with old misnamed icomplete-prompt.