bug#53629: 27.1: add option to change face `mode-line-buffer-id` when mode-line is inactive

[Juri Linkov]

>> Thanks, I think it's perfect now. It would be good to have it
>> documented, indeed.
>
> I've now added this as mode-line-window-selected-p to Emacs 29 and
> documented it in the mode line section of the manual.

Nice!  Now it's used in tab-line.el.  (I'm not sure if it can continue
using the name prefix mode-line- ?)