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/level/ChunkLoadEvent.php

@@ -31,8 +31,6 @@ use pocketmine\level\Level;
  * Called when a Chunk is loaded
  */
 class ChunkLoadEvent extends ChunkEvent{
-	public static $handlerList = null;
-
 	/** @var bool */
 	private $newChunk;
 

src/pocketmine/event/level/ChunkPopulateEvent.php

@@ -27,5 +27,5 @@ namespace pocketmine\event\level;
  * Called when a Chunk is populated (after receiving it on the main thread)
  */
 class ChunkPopulateEvent extends ChunkEvent{
-	public static $handlerList = null;
+
 }

src/pocketmine/event/level/ChunkUnloadEvent.php

@@ -29,5 +29,5 @@ use pocketmine\event\Cancellable;
  * Called when a Chunk is unloaded
  */
 class ChunkUnloadEvent extends ChunkEvent implements Cancellable{
-	public static $handlerList = null;
+
 }

src/pocketmine/event/level/LevelInitEvent.php

@@ -27,5 +27,5 @@ namespace pocketmine\event\level;
  * Called when a Level is initializing
  */
 class LevelInitEvent extends LevelEvent{
-	public static $handlerList = null;
+
 }

src/pocketmine/event/level/LevelLoadEvent.php

@@ -27,5 +27,5 @@ namespace pocketmine\event\level;
  * Called when a Level is loaded
  */
 class LevelLoadEvent extends LevelEvent{
-	public static $handlerList = null;
+
 }

src/pocketmine/event/level/LevelSaveEvent.php

@@ -27,5 +27,5 @@ namespace pocketmine\event\level;
  * Called when a Level is saved
  */
 class LevelSaveEvent extends LevelEvent{
-	public static $handlerList = null;
+
 }

src/pocketmine/event/level/LevelUnloadEvent.php

@@ -29,5 +29,5 @@ use pocketmine\event\Cancellable;
  * Called when a Level is unloaded
  */
 class LevelUnloadEvent extends LevelEvent implements Cancellable{
-	public static $handlerList = null;
+
 }

src/pocketmine/event/level/SpawnChangeEvent.php

@@ -31,8 +31,6 @@ use pocketmine\level\Position;
  * The previous spawn is included
  */
 class SpawnChangeEvent extends LevelEvent{
-	public static $handlerList = null;
-
 	/** @var Position */
 	private $previousSpawn;