A* pathfinding

Implemented a pathfinder that applies the A Star algorithm to the search
graph node, which in turn is obtained from the "active" quadtrees.
In order to have more "realistic" paths, it was decided to force the
splitting of all the active quadtrees in smaller quadtrees of size
minimumSize * minimumSize, so that all the walkable zone is subdivided
into homogeneous little blocks (quadtrees of the same depth).
This is a hybrid solution between A* on quadtrees and A* on pixels, the
latter being too expensive in terms of computation, the former returning
awkward paths in some degenerate cases.