Improve ics.Event.cmp_tuple()

[strobeflash]

What does ics.Event.cmp_tuple() stand for?

It's used for implementing the comparison operators in an easy way, see also my essay (:see_no_evil:) on ordering. I didn't really intend to make this a public API, but I guess it's a nice way of displaying the main points of an Event. Only problem is that the timestamps are given in reversed order for ToDos to sort them correctly (see the essay), so we might want to turn that into a NamedTuple (and also give the method a better name?) if we want to turn that into an actual for-the-public API.

Originally posted by @N-Coder in #340 (comment)

[strobeflash]

Low priority item decoupled from pull request.