timer id and event index are independent now

Using the index of the event array as timer id has the disadvantage that the index may be reused/reassigned to an new timer while the original index does not realize this. (example: https://github.com/JChristensen/Timer/pull/5)

My solution is to use an index-independent event-id.

To make it more clear where an event-id is expected I also introduce the type event_id.