Enumeration of possible callback event types.
BREAK event corresponds to the breaking of a defined limit on a Constraint.
ONGOING event corresponds to any step in which an interaction is occuring overlapping with the BEGIN event.
PRE event corresponds to a special mid-step event that occurs after it is determined that two objects 'will' begin to interact, but before any interaction commences.
SLEEP event corresponds to the sleeping of a Body or Constraint in the space.