BS-7901 Refactor Group Iterator

- Add class GroupIterator
- Add initialization of groupIterator in existing/new group
- Set className of iterator to Object
- Changes on the Studio UI
-- add validation on iterator name and remove the one on className
-- add iterator in Initial Value expression viewer for widgets in Group
-- Group in Form should not havea access to iterator and Group in Group should have access => does not work yet.

- Add Migration on GroupIterator