message: Improve TreeView checkboxes on DaxFormatter view
diff: @@ -14,7 +14,11 @@ namespace Sqlbi.Bravo.UI.ViewModels
  public TreeItem(MeasureSelectionViewModel parent) => _parent = parent;
  - public TreeItem(MeasureSelectionViewModel parent, TreeItem table) : this(parent) => _table = table;
  + public TreeItem(MeasureSelectionViewModel parent, TreeItem table) : this(parent)
  + {
  + _table = table;
  + IsThreeState = _table == null;
  + }
  public bool IsVisbile { get; set; } = true;
  @@ -40,13 +44,16 @@ namespace Sqlbi.Bravo.UI.ViewModels
  if (selected == 0)
  {
  _table.IsSelected = false;
  + _table.IsThreeState = false;
  }
  else if (selected == _table.Measures.Count)
  {
  _table.IsSelected = true;
  + _table.IsThreeState = false;
  }
  else
  {
  + _table.IsThreeState = true;
  _table.IsSelected = null;
  }
  }
  @@ -56,7 +63,7 @@ namespace Sqlbi.Bravo.UI.ViewModels
  }
  }
  - public bool IsThreeState => _table == null;
  + public bool IsThreeState { get; set; } = true;
  public string Name { get; set; }
  