Route quick slider preview through app core

src/node_panel_quick.h

@@ -51,6 +51,7 @@ public:
 private:
     void init_controls();
     NodeButtonCustom* init_button_brush(const std::string& name, bool szp, bool flp);
+    void update_slider_preview();
     void handle_button_brush_click(Node* target);
     void handle_button_color_click(Node* target);
 };