refactor layout.h into single file per Node* classes

engine/node_panel_color.h

@@ -0,0 +1,19 @@
+#pragma once
+#include "node.h"
+#include "node_color_quad.h"
+#include "node_slider.h"
+
+class NodePanelColor : public Node
+{
+public:
+    NodeColorQuad* m_quad;
+    NodeSliderHue* m_hue;
+    glm::vec4 m_base_color;
+    glm::vec4 m_color;
+    glm::vec2 m_cursor;
+    std::function<void(Node* target, glm::vec4 color)> on_color_changed;
+    virtual Node* clone_instantiate() const override;
+    virtual void clone_finalize(Node* dest) const override;
+    virtual void init() override;
+    void init_controls();
+};