Suppose I'm working in Interface Builder and I create an IBDesignable view called `A`. `A` contains a subview called `B`. I would like the ability to simply drag controls onto `B` in Interface Builder and have them appropriately added as children of `B`, but it looks like Interface Builder only wants me to work with `A`. Is there a way to tell `A` to instead add new subviews/controls to `B`, or to tell Interface Builder that `B` should be the "active" view?