bug#41147: A modern frame-title-format

[Lars Ljung]

I did some research and as I understand it the GNOME guidelines 
recommends just "%b", even when there is only one frame.

Many programs use the style "%b - GNU Emacs". E.g. gvim, Gimp, Firefox, 
and LibreOffice.

Either way it seems popular to place the document name first, which 
makes sense.

GNOME 2: 
https://developer.gnome.org/hig-book/3.12/hig-book.html#primary-window-titles
GNOME 3: https://wiki.gnome.org/Design/HIG/ApplicationName