Small change in Body to reuse existing logic for mass and inertia
Small Vec2 API and var names refactorings.
Enabled MouseSpringAction to catch first body by dragging, not only clicking.
Added correct calculation of center of mass of every shape instance. The defining points are translated to be lying correctly around point (0,0) as the center of mass. This also corrects the rotational behavior for complex shapes. The center of the body that is using a shape, is now always equal to the shape's center of mass.