applyForce() should reactivate the btRigidBody

[riaqn]

Bullet will deactivate dynamic rigid body after a period of time of being below a velocity threshold, to save some CPU cycle. A collision afterwards with other bodies will reactivate the body. However, when user invoke applyForce(), the object is likely to move and it should be reactivated; otherwise user won't see it move.

[riaqn]

This practice also applies to clearForces(), and all applyXXX() methods.