Starting with Python 3.8, gettext from Python's standard library finally supports the msgctxt feature of MO files. The msgctxt is used extensively by Frescobaldi, and that's the reason Frescobaldi currently uses our own MO file library in i18n/mofile.py (which works pretty well).

As soon as we require Python 3.8+, we can remove our own i18n module and use plain gettext as per the Python documentation. The MO files then should go in a standard location like <localedir>/xx_XX/LC_MESSAGES/frescobaldi.mo

(related to #1099)