Add option to automatically clean up after styleColumn / styleColumnSet when a new structure is set

CSS rules created by `Grid#styleColumn` and `ColumnSet#styleColumnSet` are managed separately from rules created by `List#addCssRule`.  Column CSS rules are now removed when `Grid#_destroyColumns` is called.  Other than when the widget is destroyed, setting new column, column set or subrow definitions will reset the column styles.  

Included are some Intern tests for `styleColumn` and `styleColumnSet`.