dispatch: add function to notify playback stopped

for the PC Speaker real driver

src/engine/dispatch.h

@@ -316,6 +316,11 @@ class DivDispatch {
      */
     virtual void notifyInsDeletion(void* ins);
 
+    /**
+     * notify that playback stopped.
+     */
+    virtual void notifyPlaybackStop();
+
     /**
      * force-retrigger instruments.
      */