SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.
* Add missing Language setting loading/saving. This was added after the
original OnionConfig PR, which is why support for it was missing.
* Change MovieConfigLoader to reuse ConfigInfos. Less duplication.
* Extract MovieConfigLoader::Save into SaveToDTM. The DTM should use
the current config and not just the movie layer. This makes more
sense than just saving the movie layer, which may not always exist,
and also fixes a crash that would happen when creating a new
recording because the movie layer wouldn't exist in that case.
(Plus, having to get the loader from the layer and call ChangeDTM
on it manually is not very pretty.)
