Add PDF writer.

src/web/xml/XmlWriter.h

@@ -1,12 +1,15 @@
 #pragma once
 
-#include "File.h"
 #include "XmlDocument.h"
+#include <string>
 
 class XmlWriter
 {
 public:
     XmlWriter() = default;
 
-    void Write(File* file, XmlDocument* document);
+    std::string ToString(XmlDocument* document);
+
+private:
+    std::string ToString(XmlElement* element, unsigned depth=0);
 };