ParticleForce

ParticleForce objects are areas which can apply directional forces to particles.

Additional notes

 * A particle object must have reactToForces set to true for the forces to work.
 * ParticleForce objects can affect particles from separate animations.
 * Point_gravity uses the center of the particleForce object as the origin of the force.
 * Wind can be thought of the same as gravity; forces are applied separately for x and y directions and the particleForce center does not matter.