FlatTreeDataGridSource should contain a constructor that allows setting of Columns property

[NeilMacMullen]

The docs contain this example code....

        Source = new FlatTreeDataGridSource<Person>(_people)
        {
            Columns =
            {
                new TextColumn<Person, string>("First Name", x => x.FirstName),
                new TextColumn<Person, string>("Last Name", x => x.LastName),
                new TextColumn<Person, int>("Age", x => x.Age),
            },
        };

However, it is NOT possible to use this approach if the type of data is not known in advance. For example, if we replace "Person" with a Row type where the number of columns may vary over time (although all rows in the table will have the same number of columns)

 public class Row(string [] rowItems)
 {
     public string this[int index]
     {
         //it's possible to get transient mismatches during update so
         //just return a dummy value if the binding is temporarily invalid
         get => index >= rowItems.Length ? string.Empty : rowItems[index];
     }
 }

then it would be desirable to write this kind of code...

var rows = result.EnumerateRows()
    .Select(items=> new Row(items))
    .ToArray();


  var columnList = new ColumnList<Row>();
  columnList.AddRange(
      result.ColumnDefinitions()
          .Select(col => new TextColumn<Row, string>(col.Name, r => r[col.Index]))
  );

 Source = new FlatTreeDataGridSource<Row>(rows,columnList) ;