Use a getter for a proxy object's instance ID

We also need this to get some random other object's instance ID, so
might as well use it everywhere.

src/common/serialization/vst3/plugin-proxy.h

@@ -212,7 +212,7 @@ class Vst3PluginProxy : public YaAudioProcessor,
         }
     };
 
-   protected:
+   private:
     ConstructArgs arguments;
 };