improve the xml serializer

while here:
- do some refactoring in BaseExpression

src/morm.h

@@ -5,7 +5,7 @@
  */
 
 /*
- * Copyright (c) 2018-2022, Tomasz Sowa
+ * Copyright (c) 2018-2023, Tomasz Sowa
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -43,9 +43,11 @@
 
 #include "jsonexpression.h"
 #include "postgresqlexpression.h"
+#include "xmlexpression.h"
 
 #include "jsonconnector.h"
 #include "postgresqlconnector.h"
+#include "xmlconnector.h"
 
 #include "modelconnector.h"
 #include "clearer.h"