FIX: back up the selected printer in prepare page

jira: none
Change-Id: Ibd163f78c4ce911a88459ecca96660ed4aa68ab2
(cherry picked from commit 392e8e18915d0db26ca8a3840d5783cc81cd1c1c)

src/slic3r/GUI/Plater.hpp

@@ -174,6 +174,7 @@ public:
     void show_SEMM_buttons(bool bshow);
     void update_dynamic_filament_list();
 
+    PlaterPresetComboBox *  printer_combox();
     ObjectList*             obj_list();
     ObjectSettings*         obj_settings();
     ObjectLayers*           obj_layers();