FIX: coPoint type couldn't display

This causes the printer setting page unable to display becase I added a
coPoint type option best_object_pos.

Add reading and writing coPoint type configs solves the problem.

Jira: STUDIO-4303

Change-Id: I34e40e567236d28ab1a7be265128275191fcc935

src/slic3r/GUI/UnsavedChangesDialog.cpp

@@ -1349,6 +1349,10 @@ static wxString get_string_value(std::string opt_key, const DynamicPrintConfig&
             opt_key == "sparse_infill_pattern",
             opt_idx);
     }
+    case coPoint: {
+        Vec2d val = config.opt<ConfigOptionPoint>(opt_key)->value;
+        return from_u8((boost::format("[%1%]") % ConfigOptionPoint(val).serialize()).str());
+    }
     case coPoints: {
         //BBS: add bed_exclude_area
         if (opt_key == "printable_area") {