Invalidate print when G-code substitution is changed

+ DiffDialog: Fixed get_string_value() for "gcode_substitution"
+ some code refactoring :
    For PresetCollection added is_independent_from_extruder_number_option(opt_key)
    to check if option is from the list of options with vector variable, which is independent from number of extruders

src/slic3r/GUI/UnsavedChangesDialog.cpp

@@ -1156,6 +1156,14 @@ static wxString get_string_value(std::string opt_key, const DynamicPrintConfig&
                 out.RemoveLast(1);
                 return out;
             }
+            if (opt_key == "gcode_substitutions") {
+                if (!strings->empty())
+                    for (size_t id = 0; id < strings->size(); id += 3)
+                        out +=  from_u8(strings->get_at(id))     + ";\t" + 
+                                from_u8(strings->get_at(id + 1)) + ";\t" + 
+                                from_u8(strings->get_at(id + 2)) + ";\n";
+                return out;
+            }
             if (!strings->empty() && opt_idx < strings->values.size())
                 return from_u8(strings->get_at(opt_idx));
         }