[ticket/11495] Do not compare to null anymore (left over from item class)

PHPBB3-11495
[ticket/11495] Explain move() more

An item is only moved up/down within the same parent. If the delta is larger
then the number of children, the item is moved to the top/bottom of the list
of children within this parent.

PHPBB3-11495
[ticket/11495] Rename fix function to regenerate_left_right_ids()

This method regenerates the left/right ids for the nested set based on the
parent/child relations. This function executes three queries per item, so
it should only be called, when the set has one of the following problems:
- The set has a duplicated value inside the left/right id chain
- The set has a missing value inside the left/right id chain
- The set has items that do not have a left/right is set

When regenerating the items, the items are sorted by parent id and their
current left id, so the current child/parent relationships are kept and
running the function on a working set will not change any orders.

PHPBB3-11495
[ticket/11495] Explain whether move_children prepends/appends/overwrittes

PHPBB3-11495
[ticket/11495] Rename set_parent to change_parent()

PHPBB3-11495
[ticket/11495] Split get_branch_data into multiple methods

PHPBB3-11495
[ticket/11495] Explain what "given item" means

PHPBB3-11495