Make it easier to build view groups

Currently, writing a new layout/view group needs a bit of boilerplate/repeated code.
This includes:
* Forwarding events to the active view
* Forwarding `call_on_any` to all views
* Forwarding `focus_view` to all views
* Handling a layout cache
* Exposing a `Vec`-like interface: add/get/remove/...

And with the upcoming `ViewPath` selector, there will be some more copy-pasted code to select the nth child.

Could we reduce the amount of duplicated code? Reduce the barrier to writing view groups correctly? Provide a more unified interface to access views? Does a `ViewGroup` trait make sense?