Migration to Qt6 done + string updates merged

[Johnny Jazeix]

Hi,

2 great improvements recently to share with the community:
* Timothée merged the update of the strings of all the activities.
Around 200 strings have been updated and the updates were mostly
proposed by Emmanuel, Anne-Françoise and Rachel (who are French
teachers) and the MR has been greatly reviewed by Timothée and Karl,
thank you all!

* We have just migrated the master branch to Qt6. There are still
potentially small issues but they will be fixed when we see them. We
don't plan to do a specific release for this version, we will wait
December/January to create the major release as usual.

The packaging has been tested for Windows, Linux and Android (+Flatpak
via the CI) for now.

With Timothée, we also tried to compile the activities as plugins
instead of using rcc files containing the qml and resources files as
we do now.
There are still a few issues on some activities and globally it works
but we won't use it for releases (at least for now). It takes a lot
more time to compile than the rcc files (I don't have precise numbers
but it's more than 10 minutes where for now we don't need 1 to create
the rcc) and the gains are minimal (very little less memory, a slight
more of CPU usage and maybe some microseconds gained here and there
but not enough to be visible).
It may be useful if we decide to allow C++ activities directly but for
now, it's not in the plans as it will burden a bit more the
maintenance and won't be compatible with the rcc way of doing.

Cheers,

Timothée & Johnny