doc: Split libgettextpo documentation into subsections
* gettext-tools/doc/gettext.texi (libgettextpo): Add a menu and split
the functions an types into separate subsections.
(Error Handling): New subsection.
(po_file_t API): New subsection
(po_message_iterator_t API). New subsection.
(po_message_t API): New subsection.
