OSX specific, related to the wxWidgets 3.1.4: Fixed wrong UI-scaling

(wxWidgets 3.1.4 can binding of wxEVT_DPI_CHANGED now, so it was called msw_rescale() for most of the controls redundantly)

+ Fixed update of the DoubleSlider after changing of the system color

src/slic3r/GUI/GUI_Preview.hpp

@@ -150,6 +150,7 @@ public:
     void refresh_print();
 
     void msw_rescale();
+    void sys_color_changed();
     void jump_layers_slider(wxKeyEvent& evt);
     void move_layers_slider(wxKeyEvent& evt);
     void edit_layers_slider(wxKeyEvent& evt);