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/EndGateway.java

@@ -3,13 +3,15 @@ package org.bukkit.block;
 import org.bukkit.Location;
 
 /**
- * Represents an end gateway.
+ * Represents a captured state of an end gateway.
  */
 public interface EndGateway extends BlockState {
 
     /**
      * Gets the location that entities are teleported to when 
      * entering the gateway portal.
+     * <p>
+     * If this block state is not placed the location's world will be null.
      * 
      * @return the gateway exit location
      */
@@ -18,6 +20,8 @@ public interface EndGateway extends BlockState {
     /**
      * Sets the exit location that entities are teleported to when
      * they enter the gateway portal.
+     * <p>
+     * If this block state is not placed the location's world has to be null.
      * 
      * @param location the new exit location
      * @throws IllegalArgumentException for differing worlds