Modernize frame-title-format: "%b - GNU Emacs"

[Stefan Kangas]

Lars Ljung proposed to change frame-title-format to something more
modern and similar to Firefox, gvim, Gimp, LibreOffice, etc.:
https://debbugs.gnu.org/41147

We currently have:

    (setq frame-title-format
          '(multiple-frames "%b"
                            ("" invocation-name "@" system-name)))

The suggestion is to change it to:

    (setq frame-title-format "%b - GNU Emacs")

Alternatively, I think one could also reasonably choose:

    (setq frame-title-format
          '(multiple-frames "%b"
                            ("%b - GNU Emacs")))

I personally think it makes sense to just do what other modern software
does here, but Eli felt that we need more opinions before making any
changes.  So please let us know if you have any thoughts.

Best regards,
Stefan Kangas