LineSegment:
-Added cones to the tips of the boxes to point in the direction the
force is facing
-BUG: When Gravity and Vector Field are the only things checked, the
Vector Field points up instead of down.

ParticlePhysics3D:
-Reset button is no longer focusable
-Added a "Repel" checkbox, makes the sphere repel instead of attract.
This is done just by multiplying the magnitude by -1 if the box is
checked.