TransitionDirector preconditions feature specification

MVP

A transition can register certain pre-conditions. If any pre-condition fails, the director will not be selected for use in the transition.

Preconditions API: Provide an API for returning preconditions.

Example pseudo-code:

TransitionDirector {
  preconditions() -> [Precondition] {
    return [Precondition({
      return elementExists
    })]
  }
}