Fix mistakes regarding i18n in !25

There were some mistakes regarding i18n in !25 (merged) (commit 957c02f9): I tested the French translation when I was working on it, but apparently, not all the subtleties of the translation system were taken into account…

I informed the translators on the mailing list: https://mail.xfce.org/pipermail/xfce-i18n/2020-September/009110.html

One thing I don't fully understand is the case of mousepad/mousepad-window.ui: it was added to po/POTFILES.in, but this does not seem to be taken into account, or sufficient. Some menubar labels are translated, but that's because they coincide with those of the toolbar, which are set in mousepad/mousepad-window.c, while the translation rate is 100% (I tested in Italian from fab8fd76).

If someone with a good knowledge of the subject (@andreldm ?) could review this, I would be grateful! 😅