main: Mark app name translatable
Also remove prgname following https://developer.gnome.org/gtk4/stable/gtk-migrating-3-to-4.html#id-1.7.4.3.15.
Also remove prgname following https://developer.gnome.org/gtk4/stable/gtk-migrating-3-to-4.html#id-1.7.4.3.15.