improve the xml serializer

while here:
- do some refactoring in BaseExpression

src/xmlconnector.h

@@ -5,7 +5,7 @@
  */
 
 /*
- * Copyright (c) 2022, Tomasz Sowa
+ * Copyright (c) 2022-2023, Tomasz Sowa
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -46,13 +46,33 @@ class XMLConnector : public FlatConnector
 
 public:
 
+	XMLConnector();
+
+	virtual void set_putting_doctype(bool put_doctype);
+	virtual void set_putting_root_element(bool put_root_element);
+	virtual bool get_putting_doctype();
+	virtual bool get_putting_root_element();
+
+	virtual void set_root_element(const wchar_t * root_name);
+	virtual void set_root_element(const std::wstring & root_name);
+	virtual std::wstring & get_root_element();
+
 	void to_text(pt::TextStream & stream, Model & model);
 
+
 protected:
 
+	bool put_doctype;
+	bool put_root_element;
+	std::wstring root_element_name;
 
 	void allocate_default_expression();
 
+	virtual void put_doctype_definition(pt::TextStream & stream);
+	virtual void put_opening_root_element(pt::TextStream & stream);
+	virtual void put_closing_root_element(pt::TextStream & stream);
+	virtual void put_root_element_name(pt::TextStream & stream);
+
 };
 
 }