cache: show background caching in task manager

Allows users to cancel and monitor total progress.

app/common/timerange.h

@@ -238,6 +238,11 @@ public:
     custom_range_ = e;
   }
 
+  int frame_index() const
+  {
+    return frame_index_;
+  }
+
 private:
   void UpdateIndexIfNecessary();
 
@@ -247,10 +252,12 @@ private:
 
   rational current_;
 
-  int index_;
+  int range_index_;
 
   int size_;
 
+  int frame_index_;
+
   bool custom_range_;
 
 };