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/inventory/CraftItemEvent.php

@@ -31,8 +31,6 @@ use pocketmine\item\Item;
 use pocketmine\Player;
 
 class CraftItemEvent extends Event implements Cancellable{
-	public static $handlerList = null;
-
 	/** @var CraftingTransaction */
 	private $transaction;
 

src/pocketmine/event/inventory/FurnaceBurnEvent.php

@@ -29,8 +29,6 @@ use pocketmine\item\Item;
 use pocketmine\tile\Furnace;
 
 class FurnaceBurnEvent extends BlockEvent implements Cancellable{
-	public static $handlerList = null;
-
 	/** @var Furnace */
 	private $furnace;
 	/** @var Item */

src/pocketmine/event/inventory/FurnaceSmeltEvent.php

@@ -29,8 +29,6 @@ use pocketmine\item\Item;
 use pocketmine\tile\Furnace;
 
 class FurnaceSmeltEvent extends BlockEvent implements Cancellable{
-	public static $handlerList = null;
-
 	/** @var Furnace */
 	private $furnace;
 	/** @var Item */

src/pocketmine/event/inventory/InventoryCloseEvent.php

@@ -27,8 +27,6 @@ use pocketmine\inventory\Inventory;
 use pocketmine\Player;
 
 class InventoryCloseEvent extends InventoryEvent{
-	public static $handlerList = null;
-
 	/** @var Player */
 	private $who;
 

src/pocketmine/event/inventory/InventoryEvent.php

@@ -31,7 +31,6 @@ use pocketmine\event\Event;
 use pocketmine\inventory\Inventory;
 
 abstract class InventoryEvent extends Event{
-
 	/** @var Inventory */
 	protected $inventory;
 

src/pocketmine/event/inventory/InventoryOpenEvent.php

@@ -28,8 +28,6 @@ use pocketmine\inventory\Inventory;
 use pocketmine\Player;
 
 class InventoryOpenEvent extends InventoryEvent implements Cancellable{
-	public static $handlerList = null;
-
 	/** @var Player */
 	private $who;
 

src/pocketmine/event/inventory/InventoryPickupArrowEvent.php

@@ -28,8 +28,6 @@ use pocketmine\event\Cancellable;
 use pocketmine\inventory\Inventory;
 
 class InventoryPickupArrowEvent extends InventoryEvent implements Cancellable{
-	public static $handlerList = null;
-
 	/** @var Arrow */
 	private $arrow;
 

src/pocketmine/event/inventory/InventoryPickupItemEvent.php

@@ -28,8 +28,6 @@ use pocketmine\event\Cancellable;
 use pocketmine\inventory\Inventory;
 
 class InventoryPickupItemEvent extends InventoryEvent implements Cancellable{
-	public static $handlerList = null;
-
 	/** @var ItemEntity */
 	private $item;
 

src/pocketmine/event/inventory/InventoryTransactionEvent.php

@@ -32,8 +32,6 @@ use pocketmine\inventory\transaction\InventoryTransaction;
  * The source of this can be a Player, entities, mobs, or even hoppers in the future!
  */
 class InventoryTransactionEvent extends Event implements Cancellable{
-	public static $handlerList = null;
-
 	/** @var InventoryTransaction */
 	private $transaction;