Become a PSR-14 provider

[weierophinney]

Q	A
BC Break	no
New Feature	yes
RFC	yes (see original ZF issue)

Description

This patch adapts EventManager to work as a PSR-14 EventDispatcher. It does so by doing the following:

• Moving all listener aggregation into classes within a ListenerProvider subnamespace; this includes both the listener attachment previously in the EventManager instance as well as the SharedEventManager instance.
• Renaming "listener aggregates" to "listener subscribers" in the new ListenerProvider subnamespace.
• Adapting EventManager to consume listener providers. By default, it will create a default priority-based listener provider, and have its various listener attachment methods proxy to that provider; it then aggregates that provider with the SharedEventManager (which is itself a provider now) in order to retrieve listeners. This is done in such a way that behavior is completely backwards compatible with current usage.
• A new named constructor, createUsingListenerProvider(), allows providing a specific listener provider for use with the EventManager. If the provider is attachment capable, the various listener attachment methods will proxy to it; otherwise, they will raise an exception.

The patch also deprecates a number of features, including:

• Most interfaces, including the EventInterface.
• The entire "shared event manager" concept (this can be accomplished via event object hierarchies instead)

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TODO

• [x] Write changelog entries
• [ ] Create documentation of new features
• [ ] Document how to start migrating to the new features in order to prepare for version 4

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Fixes #2

[chris1312]

@weierophinney @Xerkus any thoughts when this can be released?

[weierophinney]

@chris1312 I need to find time to document and create migration documentation (and/or scripts). It's a pretty big change, and I've been swamped with the migration to github actions, unfortunately.

[chris1312]

Roger that. Thanks for the update!