Added a default to EventHandler.priority()

Fixed HandlerList.unregisterAll()
Fixed incorrect isAssignableFrom check in SimplePluginManager.getRegistrationClass()

By: zml2008 <zach@zachsthings.com>

paper-api/src/main/java/org/bukkit/event/EventHandler.java

@@ -11,5 +11,5 @@ public @interface EventHandler {
 
     Class<? extends Event> event();
 
-    EventPriority priority();
+    EventPriority priority() default EventPriority.NORMAL;
 }

paper-api/src/main/java/org/bukkit/event/HandlerList.java

@@ -50,7 +50,9 @@ public class HandlerList {
 
     public static void unregisterAll() {
         for (HandlerList h : alllists) {
-            h.handlerslots.clear();
+            for (List<RegisteredListener> list : h.handlerslots.values()) {
+                list.clear();
+            }
             h.baked = false;
         }
     }