Better Popup-Placement for DataGrid-ComboBox (#1811)

* whitespace-formatting for readability (#1799)

* fixed warning about missing Grid.Column (#1799)

* fixed warning for redundant Canvas.Left=7 (#1799)

* fixed MinWidth by moving it to border  (#1799)

* simplified IsOpen-Binding (#1799)

* Attaching ComboBox-Popup to DataGridCell, compensate offset if Popup has DropShadow  (#1799)

Author: greuelpirat