Improve checking of Folder element in SyncFolderHierarchy

[idolum]

Something like this:

    //! Utility function for finding the first node of
    //! a list of names.
    //! \param elem XML node the node should retrieved from
    //! \param node_names Names of requested nodes
    //! \returns Pointer to found XML node
    inline rapidxml::xml_node<>* find_first_node_of_ns(
        const rapidxml::xml_node<>& elem,
        const Ch* namespace,
        const std::vector<std::string>& node_names)
    {
        using namespace rapidxml;

        xml_node<>* result;

        for (auto name : node_names)
        {
            result = elem.first_node_ns(namespace, name);
            if (result)
            {
                return result;
            }
        }

        return result;
    }

Then it is possible to retrieve several node types:

std::vector<std::string> elem_list = { "Folder",
    "ContactsFolder", "SearchFolder", "TaskFolder" };
const auto folder_elem = item_elem.find_first_node_of_ns(
                    internal::uri<>::microsoft::types(), elem_list);
check(folder_elem,
   "Expected <Folder>, <ContactsFolder>, <SearchFolder> or <TaskFolder> element");

Originally posted by @idolum in #156 (comment)