ensure objects are in the correct thread when loading

Since we load in a separate thread, when the QObject based objects are
instantiated, they're created with affinity to that separate thread. Now we
specifically ensure they are moved to the main thread after their creation.

app/project/item/sequence/sequence.h

@@ -62,10 +62,10 @@ public:
   virtual QString duration() override;
   virtual QString rate() override;
 
-  const VideoParams& video_params();
+  const VideoParams& video_params() const;
   void set_video_params(const VideoParams& vparam);
 
-  const AudioParams& audio_params();
+  const AudioParams& audio_params() const;
   void set_audio_params(const AudioParams& params);
 
   void set_default_parameters();
@@ -73,9 +73,6 @@ public:
 private:
   ViewerOutput* viewer_output_;
 
-  VideoParams video_params_;
-
-  AudioParams audio_params_;
 };
 
 #endif // SEQUENCE_H