cache: streamlined audio into cache task

Since we always cache audio, it makes sense to lump it into the same auto-cache
task we use for everything else.

app/task/cache/cache.h

@@ -44,6 +44,8 @@ protected:
 
   virtual void FrameDownloaded(const QByteArray& hash, const QLinkedList<rational>& times) override;
 
+  virtual void AudioDownloaded(const TimeRange& range, SampleBufferPtr samples) override;
+
 private:
   bool in_out_only_;