[GCompris] Rework of download assets way

[Johnny Jazeix]

Hi,
with Timothée, we reworked the way we download the different external
assets: background music, voices and words images.
The main reason was to allow caching of the files in the CDN and improve
the CDN hit rate.

It allowed us to update a few strings too and improve the behaviour. If you
have some time, please test the new version (the code has been merged in
master, we don't plan to provide installers for it now) as there were a lot
of use cases (some are described in
https://invent.kde.org/education/gcompris/-/merge_requests/159) and if
there are issues, the sooner we find them, the better it is.

Cheers,

Johnny
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