bug#19194: 24.4.50; `window-body-width' is not dynamic relative to font size changes

[Eli Zaretskii]

> Date: Fri, 28 Nov 2014 19:38:20 +0100
> From: martin rudalics <rudalics@gmx.at>
> CC: holtzermann17@gmail.com, 19194@debbugs.gnu.org
> 
>  >> OK.  Now how do I get the `default-font-width' from that?
>  >
>  > You need help from Emacs, because it knows everything about that
>  > font's metrics.  But we don't have an API for that for now.
> 
> I see.  Can you provide one?

Now done on trunk.