[ticket/11495] Use unique properties for the column names

PHPBB3-11495
[ticket/11495] Fix description of get_sql_where

PHPBB3-11495
[ticket/11495] Use array directly instead of phpbb_nestedset_item_interface

PHPBB3-11495
[ticket/11495] Remove item classes

PHPBB3-11495
[ticket/11495] Remove item class from unit tests

PHPBB3-11495
[ticket/11495] Fix column variable names

PHPBB3-11495