Decouple RegisteredListener and Plugin/PluginManager

[dktapps]

Description

It's currently basically impossible to use events without a Plugin context (and therefore a Server context).
This is because RegisteredListener requires a Plugin instance, even though it's only actually needed for decoration & for HandlerList::unregister().

In addition, it's non-trivial to construct a RegisteredListener, and the code needed to do so is baked into PluginManager::registerEvent(). That's before we even talk about Listener and the magic reflection detection.

Proposed changes

• Find a way to remove the plugin requirement from RegisteredListener, either by making it nullable, changing it to a more general ?object $owner, or possibly getting rid of it entirely. It's only needed for this.
• Pull the logic for PluginManager::registerEvent() out into somewhere with less dependencies (e.g. a static method in RegisteredListener or similar - enables the use of closure event handlers by core code
• Pull the logic for PluginManager::registerEvents() out into somewhere in the pocketmine\event package without Plugin or Server dependencies - enables the use of Listener magic by core code

Justification

The server core code can't listen to its own events right now without a dummy Plugin instance, which comes with a bunch of unnecessary baggage.

We also can't easily unit-test the events system, because a Plugin context and therefore a Server context is required. We could probably mock this, but it's an unnecessary inconvenience.

Alternative methods