Use unique_ptr to store the XmlNode’s children

Also fix some constness things

louloulibs/xmpp/xmpp_parser.hpp

@@ -111,6 +111,11 @@ private:
    * new children, inner or tail)
    */
   XmlNode* current_node;
+  /**
+   * The root node has no parent, so we keep it here: the XmppParser object
+   * is its owner.
+   */
+  std::unique_ptr<XmlNode> root;
   /**
    * A list of callbacks to be called on an *_event, receiving the
    * concerned Stanza/XmlNode.