Now that `Thread` keeps itself alive when it is running detached, we do
not need to hold onto it in the PulseAudio playback stream's internal
state object. This was a hack that did not work correctly because the
`Thread` object and its action `Function` would be deleted before the
action had exited and cause a crash.
This adds an abstract `Audio::PlaybackStream` class to allow cross-
platform audio playback to be done in an opaque manner by applications
in both Serenity and Lagom.
Currently, the only supported audio API is PulseAudio, but a Serenity
implementation should be added shortly as well.