Insert some modifications to implement completions highlighting.

Insert some modifications to implement completions highlighting.

This are changes needed in the minibuffer API to do highlighting from a
different package.

* lisp/minibuffer.el (minibuffer-tab-through-completions-function) : New
variable containing the action to do when pressing tab in minibuffer
and *Completions* are shown.
(minibuffer-tab-through-completions-default) : Default function value
for minibuffer-tab-through-completions-function.
(completion--in-region-1) : Modification to funcall
minibuffer-tab-through-completions-function.
(minibuffer-hide-completions-hook) : New hook to call after
closing *Completions* buffer.
(minibuffer-hide-completions) : Modify to run hook
minibuffer-hide-completions-hook.