Add virtualized indexes to itemKey function calls

[jribeiro]

Currently only the real data index is passed to the itemKey functions for Grid and List components respectively.

While this is great to be able to get an unique key for each of the rendered items, when the user scrolls, the index will change causing the new key to mismatch the previous and React to unmount and re-mount each component and, as such, recreate the DOM node.

This PR adds a new parameter to the List component itemKey function call, virtualizedIndex which represents the row index relative to the first rendered row. Similarly, for Grids, there's two new properties: virtualizedRowIndexand virtualizedColumnIndex.

Depending on what we're optimising for, using these virtualized values instead can be a performance improvement and IMO it's quite good to have the additional option mostly when the code complexity remains pretty much the same.

Relates to: #234

[fidelman]

any progress here?