Introduce a generic EventDispatcher based on PSR-14
We've been waiting for a better solution than hooks and signal/slot for a long time. PSR-14 is here and solves this for us. Plus it's compatible with Symfony5 and other frameworks (Zend).
By relying on the interface, we don't enforce a specific framework.
DI helps us to register listeners (= hook consumers), which happens outside of ext_localconf.php
Mid-term goal is to get rid of Extbase's SignalSlot dispatcher implementation (deprecation in 10 LTS would be beneficial), and long-term goal is also to not have any hooks anymore in Core, but only events.
All hooks or signal/slot implementations can be used as Listeners so there is a backwards-compatible way!