LibVideo/VideoPlayer: Dispatch state change events and update play icon

The PlaybackStateChangeEvent wasn't connected up anymore, so the player
wouldn't change icons when stopping playback due to reaching the end of
the stream or encountering an error.

Userland/Libraries/LibVideo/PlaybackManager.h

@@ -132,6 +132,7 @@ private:
     void dispatch_new_frame(RefPtr<Gfx::Bitmap> frame);
     // Returns whether we changed playback states. If so, any PlaybackStateHandler processing must cease.
     [[nodiscard]] bool dispatch_frame_queue_item(FrameQueueItem&&);
+    void dispatch_state_change();
     void dispatch_fatal_error(Error);
 
     Core::Object& m_event_handler;