MahApps/MahApps.Metro — pull request #1660 by punker76 (2014-11-20 20:45:21+00:00)
Title: New DataGridCellHelper, so the DataGridCell style can get the DataGri...

after some playing with the `DataGrid` i saw this on my output window after editong a cell...

```
System.Windows.Data Error: 4 : Cannot find source for binding with reference 'RelativeSource FindAncestor, AncestorType='System.Windows.Controls.DataGrid', AncestorLevel='1''.
BindingExpression:Path=SelectionUnit; DataItem=null; target element is 'DataGridCell' (Name=''); target property is 'NoTarget' (type 'Object')
```

some triggers fires this error at the `SelectionUnit` condition

```xml

```

so how can we get the DataGrid and fix this?

this pr adds a new helper class only for the `DataGridCell` to enable getting the DataGrid and work with it.

now it's possible to say

```xml

...

...
```