efl_ui_collection: prevent the invalid insertion of items.
the group hirachy that you can build up with group items is flattend
down in the collection, as a linear list. Based on that a user might be
able to insert items at invalid positions, like between groups, without
having set the proper group item, which ... is a problem.
This commit ensures that no new item ever destroys the neightboor hood
between two items of the same group or a item with a group and its group
Reviewers: stefan_schmidt, zmike, segfaultxavi, SanghyeonLee
Reviewed By: SanghyeonLee
Differential Revision: https://phab.enlightenment.org/D9778