What mixer is used is already logged by the code that starts the mixer.
Some mixers have additional information about what controls, etc. are
used, but in this case the only thing we add is that GStreamer is used.
This rips the mixer bits and pieces that have been hiding in the audio actor to
it's own class. The software mixer now only knows about this and nothing else
from audio.
E.g. when using pulsesink, external changes to the application-volume and
appliction-mute state will now immediately be reflected in the SoftwareMixer.
