refactor layout.h into single file per Node* classes

engine/node_popup_menu.h

@@ -0,0 +1,11 @@
+#pragma once
+#include "node.h"
+
+class NodePopupMenu : public Node
+{
+public:
+    std::function<void(Node* target, int index)> on_select;
+    virtual Node* clone_instantiate() const override;
+    virtual void init() override;
+    virtual kEventResult handle_event(Event* e) override;
+};