Mixer: add class MixerListener

Use a listener interface instead of GlobalEvents.

src/mixer/MixerInternal.hxx

@@ -25,10 +25,14 @@
 #include "thread/Mutex.hxx"
 #include "Compiler.h"
 
+class MixerListener;
+
 class Mixer {
 public:
 	const MixerPlugin &plugin;
 
+	MixerListener &listener;
+
 	/**
 	 * This mutex protects all of the mixer struct, including its
 	 * implementation, so plugins don't have to deal with that.
@@ -47,8 +51,8 @@ public:
 	bool failed;
 
 public:
-	explicit Mixer(const MixerPlugin &_plugin)
-		:plugin(_plugin),
+	explicit Mixer(const MixerPlugin &_plugin, MixerListener &_listener)
+		:plugin(_plugin), listener(_listener),
 		 open(false),
 		 failed(false) {}