Implemented a possibility to set a custom toolbars icon size (related to #2247 and same others)

src/slic3r/GUI/GUI_App.hpp

@@ -115,6 +115,7 @@ public:
     const wxFont&   normal_font()           { return m_normal_font; }
     size_t          em_unit() const         { return m_em_unit; }
     void            set_em_unit(const size_t em_unit)    { m_em_unit = em_unit; }
+    float           toolbar_icon_scale(const bool is_limited = false) const;
 
     void            recreate_GUI();
     void            system_info();