The biggest problem is: how do we deal with non-existing archives? We use Archive *Archive::create("non-existing-path") when we create a new archive (e.g. in Part and AddToArchive), but the goal is to make those create() methods private.

We could add another wrapper job, e.g. CreateJob *Archive::create(...) that internally calls AddJob. In practice:

1. If an archive already exists, we have an Archive pointer and we can just call Archive::addFiles() like we already do.
2. If we need to create a new archive, we do auto createJob = Archive::create() and then, after the job is done, we retrieve the archive (Archive *archive = createJob->archive();).