Check if an event is applicable to a Stater in current state

[gjongenelen]

We need to check if a Stater has a compatible transition for an event before actually calling Trigger(). This because we want to distinguish the 'no suitable transition-error' from actual errors occurring in the transition-callbacks (.before(), .after(), etc). The 'no suitable transition-errors' are actually acceptable in our application (and even expected).
We could not find a suitable solution for this in the project, so currently I created a fork and implemented it there (gjongenelen@c850850). It works, but I would like to know your opinion about this before I create a PR.