allow pre-scripts on end events

[burnettk]

Volker posits: "I would like to store at the end events which end state has been reached. It makes no sense to place script tasks in front of them just for this reason. This only unnecessarily inflates the process models."

there are workarounds (additional script task or post-script on the previous activity).

it probably doesn't make sense for end events to have post-scripts. therefore, this would be the only activity type that didn't have both pre and post scripts, so that might entail a bit more code/complexity in terms of implementation.