Feature: ability to compose controllers

Right now Safari doesn't support `is=` and so we cannot extend built ins, which is a shame because they include some good features which we could benefit from, for example ``. We should find a way to compose controllers in a different way.

The issue names no files, tests, or entry points. Start by tracing how Catalyst controllers are defined and attached, then investigate a composition approach that works in Safari without is=; done means controllers can be composed without that unsupported mechanism and the built-in-extension use case is covered.

Scritto dal modello di indicizzazione a partire dal testo della issue.