build: Migrate from Intltool to Gettext

Recent versions of Gettext are able to translate several formats
that are used in GNOME applications. This patch migrates from
Intltool to Gettext by using meson's i18n features.

The empty Icon= lines have been removed because they are unused,
and gettext can't handle empty values properly.

https://bugzilla.gnome.org/show_bug.cgi?id=787857