Remove usage of deprecated std::iterator

/home/elsid/dev/openmw/apps/openmw/mwlua/../mwbase/../mwworld/containerstore.hpp:264:23: warning: 'iterator<std::forward_iterator_tag, MWWorld::Ptr>' is deprecated [-Wdeprecated-declarations]
        : public std::iterator<std::forward_iterator_tag, PtrType, std::ptrdiff_t, PtrType *, PtrType&>
                      ^
/home/elsid/dev/openmw/apps/openmw/mwlua/../mwworld/inventorystore.hpp:79:36: note: in instantiation of template class 'MWWorld::ContainerStoreIteratorBase<MWWorld::Ptr>' requested here
            ContainerStoreIterator mSelectedEnchantItem;
                                   ^
/usr/bin/../include/c++/v1/__iterator/iterator.h:27:29: note: 'iterator<std::forward_iterator_tag, MWWorld::Ptr>' has been explicitly marked deprecated here
struct _LIBCPP_TEMPLATE_VIS _LIBCPP_DEPRECATED_IN_CXX17 iterator
                            ^
/usr/bin/../include/c++/v1/__config:1016:39: note: expanded from macro '_LIBCPP_DEPRECATED_IN_CXX17'
                                      ^
/usr/bin/../include/c++/v1/__config:993:48: note: expanded from macro '_LIBCPP_DEPRECATED'
                                               ^

apps/openmw/mwworld/containerstore.hpp

@@ -258,10 +258,8 @@ namespace MWWorld
             friend class MWClass::Container;
     };
 
-    
     template<class PtrType>
     class ContainerStoreIteratorBase
-        : public std::iterator<std::forward_iterator_tag, PtrType, std::ptrdiff_t, PtrType *, PtrType&>
     {
         template<class From, class To, class Dummy>
         struct IsConvertible
@@ -362,6 +360,12 @@ namespace MWWorld
         /// \return reached the end?
 
         public:
+            using iterator_category = std::forward_iterator_tag;
+            using value_type = PtrType;
+            using difference_type = std::ptrdiff_t;
+            using pointer = PtrType*;
+            using reference = PtrType&;
+
             template<class T>
             ContainerStoreIteratorBase (const ContainerStoreIteratorBase<T>& other)
             {