desktop-entry: make Icon field non-translatable?

[Jeremy Bicha]

Originally, the desktop-entry-spec set the Icon field as "string". It
was changed in 2006 to "localestring" which means it is translatable.

Recently in GNOME, many projects have switched from intltool to
gettext. gettext treats the Icon field as a translatable string
without a way for projects to disable that. In GNOME, we include a
comment to warn translators not to translate the Icon field but
translators sometimes don't see the comment. That causes breakage
because no one is providing translated icons here.

Could we please change the Icon field back to "string"?

References
----------------
https://standards.freedesktop.org/desktop-entry-spec/latest/ar01s05.html
https://standards.freedesktop.org/desktop-entry-spec/latest/ar01s06.html
https://cgit.freedesktop.org/xdg/xdg-specs/commit/?id=6855384d
https://gitlab.gnome.org/GNOME/gnome-control-center/commit/10310a198
https://savannah.gnu.org/support/?108887

Thanks,
Jeremy Bicha