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/codec/decoder.cpp

@@ -57,7 +57,7 @@ void Decoder::set_stream(StreamPtr fs)
   stream_ = fs;
 }
 
-FramePtr Decoder::RetrieveVideo(const rational &/*timecode*/, const int &/*divider*/, bool /*use_proxies*/)
+FramePtr Decoder::RetrieveVideo(const rational &/*timecode*/, const int &/*divider*/)
 {
   return nullptr;
 }
@@ -175,11 +175,6 @@ QString Decoder::GetConformedFilename(const AudioParams &params)
   return index_fn;
 }
 
-bool Decoder::ProxyVideo(const QAtomicInt *, int )
-{
-  return false;
-}
-
 bool Decoder::ConformAudio(const QAtomicInt *, const AudioParams& )
 {
   return false;