PluginManager: Move reference counting in the MediaPlugin

* Ideally we should support this feature by default to allow
future improvements to the plugins management.
* Fixes the major memory corruption that lead to various
crashes on exit in MediaPlayer.

headers/private/media/MediaPlugin.h

@@ -13,6 +13,12 @@ class MediaPlugin {
 public:
 								MediaPlugin();
 	virtual						~MediaPlugin();
+
+private:
+	// needed for plug-in reference count management
+	friend class PluginManager;
+
+			int32				fRefCount;
 };
 
 class Decoder;

headers/private/media/StreamerPlugin.h

@@ -27,10 +27,8 @@ private:
 	virtual void				_ReservedStreamer5();
 
 			MediaPlugin*		fMediaPlugin;
-			int32				fRefCount;
 			uint32				fReserved[5];
 
-	// needed for plug-in reference count management
 	friend class PluginManager;
 };