New quadtree methods, drawScene bugfixing

Added quadtree methods to:
- retrieve neighbors of the same depth at N, S, W, E, NW, NE, SW, SE
- retrieve all contained leaves
A new "item container" div's been added in order to allow a faster
change of scene (Svgs  containers and said div are simply emptied at
each scene change). This new div was necessary to fix a bug which caused
the items of the previous room to be drawn inside the new room and the
character sprite to be doubled inside the DOM.