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/DoubleSlider.hpp

@@ -192,6 +192,7 @@ public:
     ~Control() {}
 
     void    msw_rescale();
+    void    sys_color_changed();
 
     int     GetMinValue() const { return m_min_value; }
     int     GetMaxValue() const { return m_max_value; }