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.
This isn't std::moved wholesale into a member variable or further
std::moved into another function, so it's better to take it by const
reference here to avoid unnecessary reallocations of contained
std::string instances.
We can't join a detached thread, so NetPlayIndex gets deleted before
the notification thread exits, creating a race condition. We switch to
using Common::Event because just sleeping leaves the UI hung on the
thread join for a few seconds.
