Feature/Allow to set custom StateChanged event

[fmeccanici]

This PR add the functionality of registering a custom StateChanged event. I stumbled upon this myself on a project for a client, where I wanted to listen to a specific state change and apply logic for this specific state.

Since only the StateChanged event is fired, I have to add all the listeners to this event and then place if statements to check if the model is equal to the model I want the logic to apply to.

Example
In the event service provider I register StateXChangedListener to the StateChanged event. Right now I have to do this in the listener:

if ($event->model::class === ModelWithStateX::class) {
   // Apply logic for StateX state change
}

Another way is to use the updating event on the model. But this is also not ideal as I don't know which state transition has taken place.

I also saw that there is an open discussion about custom events (see #215). I am not sure if this is what they mean by customised state changed event, but for me this implementation would be very useful.

[freekmurze]

Thank you!