refactor layout.h into single file per Node* classes

engine/node_message_box.h

@@ -0,0 +1,12 @@
+#pragma once
+#include "node.h"
+#include "node_button.h"
+
+class NodeMessageBox : public Node
+{
+public:
+    Node* m_template;
+    NodeButton* btnOk;
+    virtual Node* clone_instantiate() const override;
+    virtual void init() override;
+};