[#369] Modified the quadTree implementation a bit to allow inserting entity directly

I'm not sure though on how we should manage what should be given as
parameter when inserting "children", as we need to provide the related
bounding box (me.body) and then retreive the corresponding entity
(accessible through me.body.entity)
[#369] added a debug mode for the quadtree in the debuPanel

Similar to what has been done in the ticket #103 for the region drawing
[#369] proper quadtree initialization, only populated for now once in debug mode

note : the quadtree IS NOT being used for collision, as it still need a
bit more love before being usable !