Ticket/11495 Nested Set Implementation

Ticket: http://tracker.phpbb.com/browse/PHPBB3-11495

**The interface supports:**
* moving items up/down
* set new parent
* delete/remove an item
* add/insert an item
* move all children of an item
* get a list with all parent &&/|| children
* recalculate the set (fixing broken left/right ids)