evas: Add group_member_is to smart objects

evas: Add group_member_is to smart objects

This is a new function that indicates whether an object is a child of a
parent or not. Dead simple, as this simply compares if parent == this.
Note that this check was impossible to do with the event grabber.

Also, rename group_children_iterate to group_members_iterate for
consistency with the other group_member functions.

@feature