Added templating for the mouse over when the `DataGrid` has `SelectionUnit=Cell`.
Fixes #975
Modified trigger for the cell color when selected

Corrected the issue that the active selection was not the correct color.
Unfortunately, the active selection mechanism doesn't work when the datagrid is in "selection unit cell". Therefore, I used the AccentColor for the Cell selection.
Added DataGrid example for selection unit cell
Merge pull request #1446 from Gimly/master

Fixed templating issues when the DataGrid has SelectionUnit=Cell