finalized very, very basic OTIO support

Ironed out various project and footage loading issues, and a single
cache invalidation issue.

app/render/backend/renderbackend.h

@@ -141,6 +141,8 @@ public:
     video_download_matrix_ = mat;
   }
 
+  void IgnoreNextMouseButton();
+
   static std::list<TimeRange> SplitRangeIntoChunks(const TimeRange& r);
 
 public slots:
@@ -223,6 +225,8 @@ private:
 
   QVector<QByteArray> autocache_currently_caching_hashes_;
 
+  bool ignore_next_mouse_button_;
+
 private slots:
   void WorkerFinished();