Mixer: add class MixerListener

Use a listener interface instead of GlobalEvents.

src/Partition.cxx

@@ -21,6 +21,8 @@
 #include "Partition.hxx"
 #include "DetachedSong.hxx"
 #include "output/MultipleOutputs.hxx"
+#include "mixer/Volume.hxx"
+#include "Idle.hxx"
 
 #ifdef ENABLE_DATABASE
 
@@ -47,3 +49,12 @@ Partition::SyncWithPlayer()
 {
 	playlist.SyncWithPlayer(pc);
 }
+
+void
+Partition::OnMixerVolumeChanged(gcc_unused Mixer &mixer, gcc_unused int volume)
+{
+	InvalidateHardwareVolume();
+
+	/* notify clients */
+	idle_add(IDLE_MIXER);
+}