Add nested sets implementation to phpBB core

phpBB uses nested sets to sort the the forums and subforums. Quite some MODs f.e. the gallery, use the same technic to order and nest their items (albums).

Instead of requiring new implementations for all the operations by the authors, we should implement an abstract class, that can be easily extended with the table and column names.

In a second ticket the current forum left/right id manipulations should be replaced with calls to a forum implementation of that class.

The interface should support:

• 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)

Will reuse my stuff from the gallery https://github.com/nickvergessen/phpbb3-ext-gallery-core/pull/3 (has already full test coverage)