Provide OS notifications

[tap]

Reported by tim@electrotap.com, May 19, 2008
This from Joachim Bengtsson's notes accompanying the Volumes control patch
(which was merged into version 1.3):

It could be desirable to send volume change requests as
NSDistributedNotifications so that applications using Soundflower could do
something useful with them (e.g. Airfoil using them to set the volume on
the remote host, instead of the local host).