virtualized-tabel: how to use `isActionCell`

https://issues.redhat.com/browse/CONSOLE-3747

`It turns out this behavior is supported by VirtualizedTable. The secret is to set the id of the kebab cells to an empty string ('') and their props.className to 'pf-c-table__action' . Not even remotely obvious, but it works.`

Ideally, our solution would be more obvious - or at least better documented.