make nodes more const friendly and prepare their functions for stateless work

Since the nodes won't be holding any rendering data themselves in this system,
we may as well enforce some level of non-write access by setting all the
functions to const. They were already const-friendly, they just weren't
labelled as such.

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

@@ -6,27 +6,27 @@ AudioInput::AudioInput()
   AddParameter(samples_output_);
 }
 
-Node *AudioInput::copy()
+Node *AudioInput::copy() const
 {
   return new AudioInput();
 }
 
-QString AudioInput::Name()
+QString AudioInput::Name() const
 {
   return tr("Audio Input");
 }
 
-QString AudioInput::id()
+QString AudioInput::id() const
 {
   return "org.olivevideoeditor.Olive.audioinput";
 }
 
-QString AudioInput::Category()
+QString AudioInput::Category() const
 {
   return tr("Input");
 }
 
-QString AudioInput::Description()
+QString AudioInput::Description() const
 {
   return tr("Import an audio footage stream.");
 }
@@ -36,12 +36,7 @@ NodeOutput *AudioInput::samples_output()
   return samples_output_;
 }
 
-QVariant AudioInput::Value(NodeOutput *output)
+NodeValueTable AudioInput::Value(const NodeValueDatabase &value) const
 {
-  if (output == samples_output_) {
-    // Simple passthrough from footage input
-    return footage_input_->value();
-  }
-
-  return MediaInput::Value(output);
+  return value[footage_input_];
 }