cache: better implemented changing disk cache folders

Not complete yet, but getting there.

app/codec/decoder.h

@@ -242,19 +242,13 @@ signals:
 protected:
   void SignalProcessingProgress(const int64_t& ts);
 
-  /**
-   * @brief Returns the filename for the index
-   *
-   * Retrieves the absolute filename of the index file for this stream. Decoder must be open for
-   * this to work correctly.
-   */
-  virtual QString GetIndexFilename() const = 0;
-
   /**
    * @brief Get the destination filename of an audio stream conformed to a set of parameters
    */
   QString GetConformedFilename(const AudioParams &params);
 
+  QString GetIndexFilename();
+
   bool open_;
 
   QMutex mutex_;