This has been a pain point for a long time due to the misleading nature of the name "level". It's also confusing when trying to do things like getting the XP level of the player or such, and also does not translate well to other languages.
This transition was already executed on the UI some time ago (language strings) and now it's time for the same change to occur on the API.
This will burn a lot of plugins, but they'll acclimatize. Despite the scary size of this PR, there isn't actually so many changes to make. Most of this came from renaming `Position->getLevel()` to `Position->getWorld()`, or cosmetic changes like changing variable names or doc comments.
* Event handlers always handle subclass events. public static $handlerList no longer required.
* Removed $handlerList declarations
* HandlerList cleanup: Removed HandlerList->handlers and related bake methods
* Removed obsolete Event->getHandlers()
* EventPriority: Added fromString()
* PluginManager: throw exceptions on registering handlers with invalid priorities
This allows specifying a handler of `EntityDamageEvent` which will handle any instanceof it (as per current behaviour), AND also now allows specifying a handler specifically for `EntityDamageByEntityEvent`, which only handles `EntityDamageEvent`.
This was not previously possible due to limitations in the way handlers were registered.
Abstract events may not be handled unless they declare the `@allowHandle` PhpDoc tag.
