Added a style for DataGrid
Updates to DataGrid style including styles from stefan-schweiger. Update
to match Azure DataGrid styling.
Update column header style. Add disabled style.
Update DataGrid styles to use Gray brush resources instead of color resources. Make the default DataGrid style more like stefan-schweiger's. Add Azure DataGrid
style as a separate resource.
added control to demo
missing resource
dropped AzureDataGrid style while working out what to do with it
tweaking demo layout
Merge pull request #473 from MahApps/daveraine-DataGridStyle

 DataGrid Style - Extends #446, Resolves #377