LibXML+LibWeb: Store the XML document's original source

Similar to how we store an HTML document's original source. This allows
the source to be inspected with "View Source" in the Browser.

Userland/Libraries/LibWeb/XML/XMLDocumentBuilder.cpp

@@ -46,6 +46,11 @@ XMLDocumentBuilder::XMLDocumentBuilder(DOM::Document& document, XMLScriptingSupp
 {
 }
 
+void XMLDocumentBuilder::set_source(String source)
+{
+    m_document.set_source(move(source));
+}
+
 void XMLDocumentBuilder::element_start(const XML::Name& name, HashMap<XML::Name, String> const& attributes)
 {
     if (m_has_error)

Userland/Libraries/LibWeb/XML/XMLDocumentBuilder.h

@@ -29,6 +29,7 @@ public:
     bool has_error() const { return m_has_error; }
 
 private:
+    virtual void set_source(String) override;
     virtual void element_start(XML::Name const& name, HashMap<XML::Name, String> const& attributes) override;
     virtual void element_end(XML::Name const& name) override;
     virtual void text(String const& data) override;

Userland/Libraries/LibXML/Parser/Parser.cpp

@@ -174,6 +174,7 @@ ErrorOr<void, ParseError> Parser::parse_with_listener(Listener& listener)
 {
     m_listener = &listener;
     ScopeGuard unset_listener { [this] { m_listener = nullptr; } };
+    m_listener->set_source(m_source);
     m_listener->document_start();
     auto result = parse_internal();
     if (result.is_error())

Userland/Libraries/LibXML/Parser/Parser.h

@@ -29,6 +29,7 @@ struct ParseError {
 struct Listener {
     virtual ~Listener() { }
 
+    virtual void set_source(String) { }
     virtual void document_start() { }
     virtual void document_end() { }
     virtual void element_start(Name const&, HashMap<Name, String> const&) { }