refactor layout.h into single file per Node* classes

engine/node_color_quad.h

@@ -0,0 +1,17 @@
+#pragma once
+#include "node_border.h"
+
+class NodeColorQuad : public NodeBorder
+{
+    NodeBorder* m_picker;
+    bool dragging = false;
+public:
+    glm::vec2 m_value;
+    std::function<void(Node* target, glm::vec2 value)> on_value_changed;
+    virtual Node* clone_instantiate() const override;
+    virtual void clone_finalize(Node* dest) const override;
+    virtual void init() override;
+    void set_value(float x, float y);
+    virtual kEventResult handle_event(Event* e) override;
+    virtual void draw() override;
+};