New feature: bring back gcode window as well as optimized the show/hide logic a bit

src/slic3r/GUI/GCodeViewer.hpp

@@ -697,7 +697,7 @@ public:
         GCodeWindow gcode_window;
         std::vector<unsigned int> gcode_ids;
         float m_scale = 1.0;
-
+        bool m_show_gcode_window = false;
         //BBS: GUI refactor: add canvas size
         void render(const bool has_render_path, float legend_height, int canvas_width, int canvas_height, const EViewType& view_type) const;
     };