bug#81537: 32.0.50; Fixing icomplete-in-buffer requiring users to advice-add

> We've discussed this before and Eshel and Juri suggested that the thing
> to do is make icomplete-in-buffer a proper completion-in-region-function
> value, instead of hanging off the default completion infrastructure as
> it does now.

Shouldn't the UI of icomplete-in-buffer be the same as the UI
of icomplete in the minibuffer?  That is, only when the user
types TAB, it should display the *Completions* buffer,
either in the minibuffer or in the regular buffer.
But otherwise on typing characters it should display inline
candidates immediately, without requiring to type TAB,
i.e. something like 'completion-preview-mode' already does.