bug#49844: Highlight non-selected minibuffer prompt

[Juri Linkov <juri@linkov.net>]

>> ISTR, in bug#50141 we decided to add more generalized code to windmove.el
>> that will provide 4 customizable types of window backgrounds colors
>> with different faces:
>>
>> 1. selected window
>> 2. non-selected window
>> 3. selected minibuffer window
>> 4. non-selected minibuffer window
>
> But...  isn't think pretty much unrelated to windmove, too?  I'd just
> like to have the minibuffer light up when I've moved point somewhere
> else (because it can be confusing sometimes).

This was intended to address different requests.  So far the requests
were: to highlight the background of the selected window, to highlight
the selected minibuffer window, and to highlight the non-selected
active minibuffer.  The last case actually is more complex:
the non-selected minibuffer should not be highlighted
when the Completions window is selected.  Only after moving point
out of both the minibuffer and the Completions window, the minibuffer
window should be highlighted.