Pass nodes to visitor by reference

[kduske]

Currently, node visitors take the node subclasses by pointer:

kdl::overload(
    [](const WorldNode* worldNode) {},
    [](const LayerNode* layerNode) {},
    [](const GroupNode* groupNode) {},
    [](const EntityNode* entityNode) {},
    [](const BrushNode* brushNode) {},
    [](const PatchNode* patchNode) {})

But they should take them by reference because they can never be null.