refactor layout.h into single file per Node* classes

engine/node_checkbox.h

@@ -0,0 +1,18 @@
+#pragma once
+#include "node.h"
+#include "node_border.h"
+
+class NodeCheckBox : public Node
+{
+public:
+    std::function<void(Node* target, bool checked)> on_value_changed;
+    NodeBorder* m_outer;
+    NodeBorder* m_inner;
+    bool checked = false;
+    virtual Node* clone_instantiate() const override;
+    virtual void clone_children(Node* dest) const override;
+    virtual void init() override;
+    virtual void create() override;
+    virtual kEventResult handle_event(Event* e) override;
+    virtual void draw() override;
+};