Fixed additional class names issues

- Fixed cases where the colSpec was overwritten. This happened when
  combining the class names from the columnGroups with "cellClassNames"
- Added new columnGroups option "groupHeaderCellClassName" that is set
  on all the column group header (not only specific ones), and can even be
  combined with the default generated group class names
- Fixed logic that adds first/last group column class names to not
  overwrite the colSpec