Use the configured Volume Step value for slider page steps

This makes all operations which move the volume in steps - hot keys,
mouse wheel over the system tray icon, and slider page steps - use the
same step value.

Improve the interface between the KMix application and the Volume class,
by adding Volume::setVolumeStep() to set the value.  There is no need for
a getter function since a value can be obtained from Volume::volumeStep().
Eliminates some public static variables that looked like constants.

Add an explanatory tool tip to the Volume Step configuration spin box.