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/output/timeline/timeline.cpp

@@ -35,7 +35,6 @@ TimelineOutput::TimelineOutput()
   for (int i=0;i<kTrackTypeCount;i++) {
     // Create track input
     NodeInput* track_input = new NodeInput(QString("track_in_%1").arg(i));
-    track_input->set_data_type(NodeParam::kTrack);
     AddParameter(track_input);
     track_inputs_.replace(i, track_input);
 
@@ -53,58 +52,61 @@ TimelineOutput::TimelineOutput()
   AddParameter(length_output_);
 }
 
-Node *TimelineOutput::copy()
+Node *TimelineOutput::copy() const
 {
   return new TimelineOutput();
 }
 
-QString TimelineOutput::Name()
+QString TimelineOutput::Name() const
 {
   return tr("Timeline");
 }
 
-QString TimelineOutput::id()
+QString TimelineOutput::id() const
 {
   return "org.olivevideoeditor.Olive.timeline";
 }
 
-QString TimelineOutput::Category()
+QString TimelineOutput::Category() const
 {
   return tr("Output");
 }
 
-QString TimelineOutput::Description()
+QString TimelineOutput::Description() const
 {
   return tr("Node for communicating between a Timeline panel and the node graph.");
 }
 
-QVector<TrackOutput *> TimelineOutput::Tracks()
-{
-  return track_cache_;
-}
-
-NodeOutput *TimelineOutput::length_output()
-{
-  return length_output_;
-}
-
-const rational &TimelineOutput::timeline_length()
+const rational &TimelineOutput::length() const
 {
   return length_;
 }
 
-const rational &TimelineOutput::Timebase()
+const QVector<TrackOutput *>& TimelineOutput::Tracks() const
+{
+  return track_cache_;
+}
+
+NodeOutput *TimelineOutput::length_output() const
+{
+  return length_output_;
+}
+
+const rational &TimelineOutput::timeline_length() const
+{
+  return length_;
+}
+
+const rational &TimelineOutput::timebase() const
 {
   return timebase_;
 }
 
-QVariant TimelineOutput::Value(NodeOutput *output)
+NodeValueTable TimelineOutput::Value(const NodeValueDatabase &value) const
 {
-  if (output == length_output_) {
-    return QVariant::fromValue(length_);
-  }
-
-  return 0;
+  NodeValueTable table = value.Merge();
+  table.Push(NodeParam::kRational, QVariant::fromValue(length()));
+  return table;
 }
 
 void TimelineOutput::UpdateTrackCache()
@@ -150,12 +152,12 @@ void TimelineOutput::SetTimebase(const rational &timebase)
   emit TimebaseChanged(timebase_);
 }
 
-NodeInput *TimelineOutput::track_input(TrackType type)
+NodeInput *TimelineOutput::track_input(TrackType type) const
 {
   return track_inputs_.at(type);
 }
 
-TrackList *TimelineOutput::track_list(TrackType type)
+TrackList *TimelineOutput::track_list(TrackType type) const
 {
   return track_lists_.at(type);
 }