Changed to send completed event message from main thread (#22)

I changed to send the completed event messages from main thread to avoid flutter engine's fml checker error.

packages/video_player/elinux/gst_video_player.h

@@ -65,6 +65,7 @@ class GstVideoPlayer {
   double playback_rate_ = 1.0;
   bool mute_ = false;
   bool auto_repeat_ = false;
+  bool is_completed_ = false;
   std::shared_mutex mutex_buffer_;
   std::unique_ptr<VideoPlayerStreamHandler> stream_handler_;
 };