removed all dependence on NodeOutputs

If Nodes only have the one output, we don't need to do so much differentiation
between them. Previous iteration used outputs as like a distinct function
within a Node (e.g. length output would return one result, buffer output would
produce a different result - each run different code to produce their results).
Now in this iteration, it's more accurate to say a Node is just one function
(which seems more appropriate for a node system anyway).

app/node/output/track/track.h

@@ -60,6 +60,9 @@ public:
 
   Block* NearestBlockAfter(const rational& time) const;
 
+  Block* BlockAtTime(const rational& time) const;
+  QList<Block*> BlocksAtTimeRange(const TimeRange& range) const;
+
   const QVector<Block*>& Blocks() const;
 
   virtual void InvalidateCache(const rational& start_range, const rational& end_range, NodeInput* from = nullptr) override;