[DataGrid] Keyboard navigation broken on page > 0

[flaviendelangle]

Introduced by #2994, was never released, I will merge it before the next release if no review 👍

The variable wording was approximate and I did not test enough.
rowCount was refering to the index of the last element of the page + 1

A few examples:

• rows.length: 100, page: 0, pageSize: 10 => rowCount: 10
• rows.length: 100, page: 1, pageSize: 10 => rowCount: 20
• rows.length: 14, page: 1, pageSize: 10 => rowCount: 14

I will probably clean those methods later to add explicit comments on each instruction.
Because // we go to the current row, first col, or last col! is not very explicit to me.