add template Node::clone() and remove destroy_immediate

src/node_stroke_preview.h

@@ -54,6 +54,6 @@ public:
     void draw_stroke_immediate();
     virtual void draw() override;
     virtual void handle_resize(glm::vec2 old_size, glm::vec2 new_size, float zoom) override;
-    virtual void destroy_immediate() override;
+    virtual void destroy() override;
     virtual void handle_on_screen(bool old_visibility, bool new_visibility) override;
 };