[bugfix] Fix typedef for dereferencing an ConstArrayListIterator

The type returned when dereferencing an iterator should correspond to the typedef named reference even if the iterator acts on a const container. For the ArrayList::const_iterator this was not the case so far and reference was a non-const value_type& which is fixed by this patch. While not beeing part of the standard iterator interface, the old typedef const_reference is kept for backward compatibility and consistency with ArrayList::iterator.