Added features

• Added columnGroups.
• Added columnHeight, columnFilterHeight.

Note to users of previous versions

• Changed the default value of enableGridBorderShadow from true to false.
• Changed interface of toggleSortColumn, sortAscending, sortDescending, sortBySortIdx methods.

  void toggleSortColumn(Key columnKey); // less than 2.8.0
  void toggleSortColumn(PlutoColumn column); // 2.8.0 or later

  void sortAscending(PlutoColumn column); // less than 2.8.0
  void sortAscending(PlutoColumn column, {bool notify = true}); // 2.8.0 or later

  void sortDescending(PlutoColumn column); // less than 2.8.0
  void sortDescending(PlutoColumn column, {bool notify = true}); // 2.8.0 or later

  void sortBySortIdx(); // less than 2.8.0
  void sortBySortIdx(PlutoColumn column, {bool notify = true}); // 2.8.0 or later