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.
Lioncash