This patch allows grade items with itemnumber > 0 being edited in the
gradebook. It also improves the grade_verify_idnumber() so that it does
not allow the item with itemnumber > 0 have the same idnumber as the
major grade item with itemnumber 0.
We should provide a common approach to float grade values so modules
will be consistent in grading algorithm. That is why I push it here.
Note that basically this complements grade_floats_different().
Used in Workshop 2.0.