Improvements to BlockStates

* Update Javadoc of block state classes to better match implementation behaviour.
* Add Container interface which provides access to the snapshot inventory.

By: Lukas Hennig <lukas@wirsindwir.de>

paper-api/src/main/java/org/bukkit/block/Skull.java

@@ -1,11 +1,10 @@
 package org.bukkit.block;
 
-import java.util.UUID;
 import org.bukkit.OfflinePlayer;
 import org.bukkit.SkullType;
 
 /**
- * Represents a Skull
+ * Represents a captured state of a skull block.
  */
 public interface Skull extends BlockState {