SelectedItem(s) should be persisted when paginating or sorting

[symbiogenesis]

The current selection logic is based on the internal CollectionView.

This implementation is elegant and maintainable, but loses selected items when paginating. Or when sorting while pagination is enabled.

It isn't always obvious that they should be persisted when paginating, but it does seem obvious if the SelectedItem(s) are being driven by a ViewModel that they should be.

There could be an argument to re-engineer it entirely. But it seems like a massive overhaul because we couldn't just bind to the CollectionView or rely on its SelectionChanged event at all.

Perhaps the best approach would be to cache the SelectedItem(s) prior to sorting or paginating, and then restore them.

I should be able to make this happen.