This seems to work quite well including hotplugging of devices, changes to
default sinks/sources etc.
So far I only provided percentage values for sink/source since they seem most
useful. Defaults are set to DEFAULT_SINK/DEFAULT_SOURCE which means it will
follow whatever defaults are at the moment (instead of being fixed to a given
device).
One thing currently left unhandled is when pulseaudio/pipewire gets
disconnected/restarted. In such a case we mark the sink/source as offline but
there is no reconnect attempted later.
Added man page + more complex example which includes mixed usage of both input
and output.
