The ComposableController now supports controllers that are based upon
BaseControllerV2. These newer controllers use the controller messaging
system to communicate, so the messenger is now a required constructor
parameter if any BaseControllerV2 controllers are used.

The `name` property in `BaseControllerV2` had to be made public for the
ComposableController to properly construct its state. This seems OK. It
was only kept private initially until a use was found for it.

These generic type parameters weren't really used in practice. They
were passed to the `messagingSystem`, but we erased the type of the
messaging system in the one place that we call it, making the more
specific types pointless. You can tell that they weren't useful because
they were never set in tests, and everything still worked.