Remove redraw callback when destroying

- ColReorder's redraw callback was getting called sometimes after it has
  been destroyed, and this caused an error.  For instance, as DataTable
  gets destroyed, it first calls all the destroy callbacks, then tries
  to restore column visibility, but at that point, ColReorder's redraw
  callback gets triggered.
Added fnGetCurrentOrder API for aiOrder equiv data

- Without such API, the fnReorderCallback is not so useful.  Now, we can
  access the new column order array within the callback as follows:

	this.fnGetCurrentOrder()