The label of column header.
Typically, this will be Text widget. You can also set Icon (Typically using size 18), or a Row with an icon and Text.
If you want to take the entire space for widget, e.g. when you want to use Center, you can wrap it with an Expanded.
The widget will be loaded in text area alone. When sorting is applied, the default sort icon will be loaded along with the widget.
final Widget label;