implemented support for multilayered keyframes

This adds the ability to keyframe more than one value per input (e.g. a vec2,
vec3, etc.) so that you can animate, for example, an X axis separately from a
Y axis.

app/node/keyframe.cpp

@@ -37,7 +37,7 @@ NodeKeyframePtr NodeKeyframe::Create(const rational &time, const QVariant &value
 
 NodeKeyframePtr NodeKeyframe::copy() const
 {
-  NodeKeyframePtr copy = std::make_shared<NodeKeyframe>(time_, value_, type_);
+  NodeKeyframePtr copy = std::make_shared<NodeKeyframe>(time_, value_, type_, track_);
   copy->bezier_control_in_ = bezier_control_in_;
   copy->bezier_control_out_ = bezier_control_out_;
   return copy;
@@ -116,6 +116,11 @@ void NodeKeyframe::set_bezier_control(NodeKeyframe::BezierType type, const QPoin
   }
 }
 
+const int &NodeKeyframe::track() const
+{
+  return track_;
+}
+
 NodeKeyframe::BezierType NodeKeyframe::get_opposing_bezier_type(NodeKeyframe::BezierType type)
 {
   if (type == kInHandle) {