Fixed templating issues when the DataGrid has SelectionUnit=Cell

This pull request fixes the issue #975 that made the selected cells disappear when the `DataGrid` cells could be selected individually (`SelectionUnit = Cell`).

It simply adds a Setter for the `Background` property of the cell with the accent color when it is selected.

I've also changed the way the mouse over is displayed when in that mode, it only highlights one cell instead of highlighting the whole row.

Fixes #975