implemented core UI caching feedback functionality

Not a perfect implementation yet, but this shows UI feedback on what frames are
cached and which ones aren't.

app/render/backend/videorenderframecache.h

@@ -41,6 +41,8 @@ public:
 
   void RemoveHashFromCurrentlyCaching(const QByteArray& hash);
 
+  QList<rational> FramesWithHash(const QByteArray& hash);
+
 private:
   QMap<rational, QByteArray> time_hash_map_;