Implement volume and rate on OS X

Change-Id: I44b4ac64e6b0d9ce921fb8da77e809088a31f754
Reviewed-by: Laurent Montel <laurent.montel@kdab.com>
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@theqtcompany.com>
Add basic tests

Change-Id: I2c671f04fe19f74f7b781a12bd23ad10a15fc2bb
Reviewed-by: Jan Arve Sæther <jan-arve.saether@theqtcompany.com>