Remove selectedRow prop from DataTable and add a new prop to toggle multi-row selection

[acelaya]

#1360 introduced support for multi-row selection, by adding a new selectedRows prop.

With those changes, DataTable allows multi-row selection only if selectedRows is provided, and single-row selection if selectedRow is instead. However, we have not explicitly handled what happens if both are provided, other than just preferring selectedRows over selectedRow.

To make the props API more obvious, let's remove selectedRow, and add a new prop to enable/disable multi-row selection.