[AFV] Ref T730, improved volume handling

* for the asymmetric output volume -60/18dB, make sure that normalized 50 means 0dB
* slider is centered
* return value if value for volume was changed

src/blacksound/sampleprovider/volumesampleprovider.cpp

@@ -33,5 +33,12 @@ namespace BlackSound
             }
             return samplesRead;
         }
+
+        bool CVolumeSampleProvider::setVolume(double volume)
+        {
+            const bool changed = !qFuzzyCompare(m_volume, volume);
+            m_volume = volume;
+            return changed;
+        }
     } // ns
 } // ns

src/blacksound/sampleprovider/volumesampleprovider.h

@@ -32,7 +32,7 @@ namespace BlackSound
 
             //! Volume @{
             double volume() const { return m_volume; }
-            void setVolume(double volume) { m_volume = volume; }
+            bool setVolume(double volume);
             //! @}
 
         private: