use NodeInputArray for tracks as well

These were introduced for the new Block system and make just as much sense for
the Track system. They've now been implemented for both.

app/node/inputarray.h

@@ -22,7 +22,9 @@ public:
 
   int IndexOfSubParameter(NodeInput* input) const;
 
-  NodeInput* ParamAt(int index) const;
+  NodeInput* First() const;
+  NodeInput* Last() const;
+  NodeInput* At(int index) const;
 
   const QVector<NodeInput*>& sub_params();