improve the xml serializer

while here:
- do some refactoring in BaseExpression

src/baseexpression.cpp

@@ -241,6 +241,21 @@ void BaseExpression::put_field_name(const wchar_t * field_name, const FT & field
 }
 
 
+void BaseExpression::put_field_closing_name(const wchar_t * field_name, const FT & field_type, ModelEnv * model_env)
+{
+	put_field_name(field_name, field_type, model_env);
+}
+
+
+void BaseExpression::put_value_list_opening_index(size_t index, const FT & field_type)
+{
+}
+
+
+void BaseExpression::put_value_list_closing_index(size_t index, const FT & field_type)
+{
+}
+
 
 void BaseExpression::save_foreign_key(const wchar_t * field_name, const FT & field_type, ModelEnv * model_env)
 {