node: split off traversing functions from renderer for usage elsewhere

Fixes issue where Viewer node wouldn't pick up correct length if a tracks
weren't in use.

app/node/input/media/audio/audio.cpp

@@ -28,8 +28,3 @@ QString AudioInput::Description() const
 {
   return tr("Import an audio footage stream.");
 }
-
-NodeValueTable AudioInput::Value(const NodeValueDatabase &value) const
-{
-  return value[footage_input_];
-}

app/node/input/media/audio/audio.h

@@ -15,9 +15,6 @@ public:
   virtual QString Category() const override;
   virtual QString Description() const override;
 
-protected:
-  virtual NodeValueTable Value(const NodeValueDatabase& value) const override;
-
 private:
 };