bug#13602: 24.3.50; remove bindings for `icomplete-minibuffer-map' - make a separate mode

[Drew Adams]

> > It is not kosher to bind such keys in the minibuffer in a 
> > general mode.
> 
> Of course it is.  Most modes do that.

Icomplete is not a new mode.  Its use has been to provide informative help.  It
has never interfered with minibuffer bindings in its long history.

Even if Icomplete were new, we should separate the binding of keys out, making
that optional.  It should be easy to use Icomplete with all the existing
minibuffer bindings.

Introducing alternative minibuffer bindings for use with Icomplete mode is fine.
But they should be optional.

> > Let users bind them if they like.
> 
> The user is still, as always, free to add or remove bindings.
> E.g. you might like to add (setq icomplete-minibuffer-map 
> (make-sparse-keymap))
> to your .emacs if you prefer the old behavior.

Make it _easier_ on users.  Do what we've done for CUA mode and CUA selection
mode.

Let users easily separate the informative help aspect of Icomplete - what it has
always provided, from the addition of Ido-like cycling.

And do not have Icomplete trample minibuffer bindings by default.  Let users opt
in for that.