Support multiple types within an object for custom group

Allow custom groups to extend zero or more "types" of the same object. Thus a custom group can extend one or more specific events (custom group via event name)

Check: http://civicrm.org/node/644 for specific details

Custom group settings screen should detect add vs update mode. Adding more subtypes should always be allowed (if any) while updating (removing) any subtype depends on - if there is any existing custom data information for that particular subtype.