[#369] quadtree starting to work (but for now only in debug mode)

- the quadtree library now call the items getBounds() function to get
their relative regions
- The quadtree is now only populated with object having a body property
(that limit it to entities)
- Update the default max depth value to match the default one in the
library

Note :
- the retrieve functions still returns much more object that I would
actually expect.
- the viewport should also be passed to initialize the quadtree and not
the whole world (learning as I go)