This is a pointer type that refers to an iterator that produces a sequence of messages.
po_message_iterator returns an iterator that will produce the
messages of file that belong to the given domain. If domain
NULL, the default domain is used instead. To list the messages,
use the function
po_message_iterator_free function frees an iterator previously
allocated through the
po_next_message function returns the next message from
iterator and advances the iterator. It returns
NULL when the
iterator has reached the end of its message list.