implement entire project in nodes

Project items are now represented directly in nodes opening up more versatility and possibilities. This is the first iteration of this and will be buggy. Need to test thoroughly.

app/render/playbackcache.cpp

@@ -151,12 +151,7 @@ Project *PlaybackCache::GetProject() const
     return nullptr;
   }
 
-  Sequence* sequence = static_cast<Sequence*>(viewer->parent());
-  if (!sequence) {
-    return nullptr;
-  }
-
-  return sequence->project();
+  return viewer->project();
 }
 
 void PlaybackCache::RemoveRangeFromJobs(const TimeRange &remove)