Ability for user to get notifications on what is being displayed.

I would like an action `sliceDidChange` (or something like that) where a user could learn what is actually being displayed by the `ember-collection`. When the collection changes, the callback gets called with "startingIndex" and "visibleCount" giving offset and limit of slice from collection being displayed. The use cases to for this include:

1) take an action when content is actually displayed
2) get more content when content is almost displayed

@BryanCrotaz and I need this functionality in https://github.com/shaunc/ember-grid to get content ready for display so we can have smooth scrolling. It would also be useful for smooth infinite scroll (request from server when near), or to lazily update content when its not ready yet.

[PR coming shortly]

The issue does not name files or tests. Start by tracing ember-collection's rendering lifecycle and existing callback APIs; confirm how visible slices change during scrolling. Done means a user-facing callback reports startingIndex and visibleCount whenever the displayed collection slice changes.

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