Implemented event handler inheritance, allow registering handlers for any valid event (#1792)

* 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.

src/pocketmine/event/entity/EntityDamageByEntityEvent.php

@@ -31,7 +31,6 @@ use pocketmine\entity\Living;
  * Called when an entity takes damage from another entity.
  */
 class EntityDamageByEntityEvent extends EntityDamageEvent{
-
 	/** @var int */
 	private $damagerEntityId;
 	/** @var float */