Provide better support for index handling for consumers of ListChangeEvent

Consumers of `ListChangeEvent` need to handle both the case of an `int` event index and a `slice` event index, typically leading to code that's branchy, awkward, and hard to test thoroughly, as well as requiring understanding of how the event index is normalised. Traits could help here by providing some covenience methods or properties on the `ListChangeEvent` object to allow easy retrieval of list indices.

An example use-case is needing both the indices and the values of removed or added entries, in a way that works regardless of whether `event.index` is an integer or a slice.