FIX: back up the selected printer in prepare page

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

src/slic3r/GUI/Plater.cpp

@@ -2773,6 +2773,11 @@ void Sidebar::update_dynamic_filament_list()
     dynamic_filament_list_1_based.update();
 }
 
+PlaterPresetComboBox* Sidebar::printer_combox()
+{
+    return p->combo_printer;
+}
+
 ObjectList* Sidebar::obj_list()
 {
     // BBS