Reworked sys_color_changed() functions

Fixed OSX specific bugs:
- toolbar flashing for some mainframe sizes (Retina specific)
- size of mainframe when settings layout in slNew mode

Added missed icons to the "white" folder

src/slic3r/GUI/GUI_App.hpp

@@ -138,6 +138,7 @@ public:
     const wxFont&   bold_font()             { return m_bold_font; }
     const wxFont&   normal_font()           { return m_normal_font; }
     int             em_unit() const         { return m_em_unit; }
+    wxSize          get_min_size() const;
     float           toolbar_icon_scale(const bool is_limited = false) const;
     void            set_auto_toolbar_icon_scale(float scale) const;