Teach EnderDragon how to throw EntityExplosionEvents when it breaks blocks

We also teach CraftWorld how to explode an event, taken from
Explosion.a(boolean) (the code that breaks blocks and drops them on the ground).

The EnderDragon has a flag that slows it down when it hits unbreakable blocks:
Obsidian, White Stone or Bedrock. It might be useful to extend the event so that
plugins can set this.

Letting the API set the default yield for an explosion event has been
deprecated, so we now set the default yield using the appropriate constructor.

src/main/java/net/minecraft/server/Explosion.java

@@ -185,7 +185,7 @@ public class Explosion {
             }
         }
 
-        EntityExplodeEvent event = new EntityExplodeEvent(explode, location, blockList);
+        EntityExplodeEvent event = new EntityExplodeEvent(explode, location, blockList, 0.3F);
         this.world.getServer().getPluginManager().callEvent(event);
 
         arraylist.clear();