Change sound volume in dB, not in pseudo-percent

[scytho]

See mate-desktop/mate-desktop#223. Steps of 2dB would be way to big. 0.5dB would be good, as that is about the minimal change the human ear can detect. If that appears to result in too many steps, an option to configure the step size would be nice.

[lukefromdc]

This should be moved to mate-media, the package providing the volume control applet and status icon