Show completions category in heading line

Show completions category in heading line

Extend 'completions-header-format' with a placeholder for the
completions category.  Stop hard-coding the 'shadow' face for the
heading line default value, instead define a derived face.

* lisp/minibuffer.el (completions-header-format): Extend.
(completions-heading): New face.
(completion-category): New variable.
(minibuffer-completion-help): Let-bind it.
(display-completion-list): Use it.

* doc/emacs/mini.texi (Completion Options): Elaborate.

* etc/NEWS: Announce.