Add forward method to FactoryVecDeque objects

[chriskilding]

As I've previously mentioned I am looking at how to do state management in Relm apps with complex nested view models.

If you imagine a view hierarchy of App > Foo, and you are only using SimpleComponents, you can effectively bubble events up the view stack like this (and when it reaches the top, you can then forward AppOutput events into a headless document component that does loading/saving):

let foo: Controller<Foo> = Foo::builder()
  .launch(something)
  .forward(sender.output_sender(), |msg| match msg {
    FooOutput::A => AppOutput::A,
    FooOutput::B => AppOutput::B
  });

But to make this approach viable, we also need the ability to do this with Factory components (Vecs, VecDequeues, Hashmaps). So we need something like:

let foo: FactoryVecDeque<Foo> = FactoryVecDeque::builder()
  .launch(component_iter, widget)
  .forward_from_children(sender.output_sender(), |msg| match msg {
    FooOutput::A(index) => AppOutput::A(index)
  });

Is this the right user-facing DSL / API to achieve this? And if so, where in the Relm4 code should the builder() be added to FactoryVecDeque? (I am happy to contribute a PR for this if you can give me the guidance.)

[chriskilding]

The essence of this has now been implemented in #523

FactoryVecDeque (and FactoryHashMap) are built using a builder API, which is fairly close to the prototype above. They can now forward events from their FactoryComponent children, just as a regular SimpleComponent can.

See https://github.com/Relm4/Relm4/blob/273e167b1751ca122c3846ed8db6c8b19f901765/examples/state_management.rs for an example of how to use it.