project: replaced proxy with pre-cache

Removed proxy task and replaced with a true honest-to-god pre-cache for
footage. This footage is pre-cached to a sequence and therefore 100% ready
for use in it once the task is done.

app/task/cache/cache.h

@@ -36,10 +36,9 @@ public:
             const AudioParams &aparams,
             bool in_out_only);
 
-public slots:
+protected:
   virtual bool Run() override;
 
-protected:
   virtual QFuture<void> DownloadFrame(FramePtr frame, const QByteArray &hash) override;
 
   virtual void FrameDownloaded(const QByteArray& hash, const std::list<rational>& times) override;