[ticket/11495] Add basic interface with nestedset operations

PHPBB3-11495
[ticket/11495] Add abstract implementation of the interface

PHPBB3-11495
[ticket/11495] Add forum implementation of nestedset

PHPBB3-11495
[ticket/11495] Add unit tests for the implemented functions

PHPBB3-11495
[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
[ticket/11495] Move nestedset default values to new method

PHPBB3-11495
[ticket/11495] Manually specify the table columns

PHPBB3-11495
[ticket/11495] Remove item class as its no longer required

PHPBB3-11495
[ticket/11495] Acquire locks for operations that manipulate the tree

PHPBB3-11495
[ticket/11495] Remove acquire locks from forum implementation

PHPBB3-11495
[ticket/11495] Fix Spacing and lowercase on docs

PHPBB3-11495
[ticket/11495] Use item_id only as parameter for get_branch_data()

PHPBB3-11495
[ticket/11495] Use item_id only as parameter for delete() and remove()

The data is acquired again anyway

PHPBB3-11495
[ticket/11495] Cast some values to int

PHPBB3-11495
[ticket/11495] Use item ids instead of requiring all data

The data is grabbed again in most cases anyway, so it just makes the system
easier to use.

PHPBB3-11495