add mode for copying values only and not connections

Sometimes we'll be copying nodes without wanting to copy their connections,
since we'll want to connect them to equivalent copies rather than connecting
them to the same nodes the originals were connected to. We now have an extra
parameter to distinguish such operations.

app/node/input.cpp

@@ -280,7 +280,7 @@ void NodeInput::set_maximum(const QVariant &max)
   has_maximum_ = true;
 }
 
-void NodeInput::CopyValues(NodeInput *source, NodeInput *dest)
+void NodeInput::CopyValues(NodeInput *source, NodeInput *dest, bool include_connections)
 {
   // Copy values
   dest->keyframes_ = source->keyframes_;
@@ -289,7 +289,7 @@ void NodeInput::CopyValues(NodeInput *source, NodeInput *dest)
   dest->set_is_keyframing(source->is_keyframing());
 
   // Copy connections
-  if (source->get_connected_output() != nullptr) {
+  if (include_connections && source->get_connected_output() != nullptr) {
     ConnectEdge(source->get_connected_output(), dest);
   }
 }