Automatically creating categories for modules with multiple grade_items

The problem with grade_create_category() is that it breaks the idea that modules should not know about the gradebook implementation. Direct access to grade categories might cause problems later.

Solution A:

• add idnumber to grade categories
• add new parameter to grade_update $category=null which would specify requested category idnumber, name and aggregation
  We should not IMHO use category id /internal implementation) or name (might change) - the id could solve the identification problem

Solution B:

• remove grade categories from API - modules could not specify them
• use new "Grading" tab in module editing page - similar to roles overrides; mods would not know about this code, teachers could administer all grade items attached to mod there