Removed VolumeChanged signal, as ConfigChanged will trigger what is needed.
Only applies UpdateSoundStream to things that can change during emulation.
Settings::SetVolume might no longer be used, but left it in.
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.