The goal here is to ensure we check for audio backends in a way that
makes sense. On macOS, let's just always use Audio Unit (and thus avoid
any checks for Pulse, to reduce needless/confusing build log noise). We
will also only use the Qt audio backend if no other backend was found,
rather than only checking for Pulse.
Instead of having to duplicate the audio stream backend conditions, just
define PlaybackStream::create in each audio backend implementation file.
We provide a weak definition in PlaybackStream.cpp as the fallback.
