Account for virtualization in the CollectionView

[symbiogenesis]

The CollectionView control we use implements virtualization, and this is likely the cause of recent erroneous behavior seen in bugs such as #166, #170, and #172.

Basically, virtualization is a concept where under the hood the CollectionView is able to re-use the individual views inside of it, which in our terms would be the rows. It sort of pools them and re-uses them as you scroll, instead of fully creating and destroying them.

Ideally we should come up with a solution to this problem, which may involve such things as producing a unique identifier for the rows, or keeping track of all the Rows in a collection, or effectively implementing our own virtualization system.

Having access to the rows from the DataGrid class itself would be a nice convenience for maintenance, although that isn't the only possible approach.

This is just a placeholder for discussion and investigation. I do not know the right answer.

I do not strictly know why behavior changed after 8.0.7 or why we need to clear out the cells in the rows all the time now, or why the row indexes aren't reliable anymore.