ignore/delete older translations on update/startup

esp. useful with nightly builds and self built versions, saves us from having to manually delete translations that had been updated for an earlier build, but are not used because the 'make lrelease' step creates and the 'make install' step bundles older versions but with a newer timestamp, so that these take precedence when loading the translation.
Probably only one of the commits is really needed... but I'd prefer to have'em both, as one works on MuseScore start, for the current set up locale and the other works on update, for all locales.