i18n: Update POT and PO files

[HuidaeCho]

# remove locally installed addons to avoid message strings being scanned
grass  --tmp-project XY --exec g.extension.all -f operation=remove

# create POT file and update PO files
cd locale/
make pot
make update-po

# bulk of potentially duplicated gettext messages
cd po/
for i in `ls grass*_*.po` ; do
    msguniq --use-first --width=80 $i > tmp.$i
    rm -f $i
    mv tmp.$i $i
done

(for posterity: edited for de-duplication and removal of locally installed addons )

[neteler]

Thanks! And important for the upcoming 8.4.0 release

[neteler]

(I have made an attempt to update the i18N procedure, see PR description above)