quadtree: Added quadtree lib

Adding an absctract class to generate a quadtree according its needs

Signed-off-by: Stefan Lage <lagestfan@gmail.com>
quadtree: Added an implementation of quadtree for scene2d

Signed-off-by: Stefan Lage <lagestfan@gmail.com>
quadtree: Add an example

Reedit the example 'Leapfrog' using the quadtree

signed-off by: Stefan Lage <lagestfan@gmail.com>