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/Container.java

@@ -0,0 +1,36 @@
+package org.bukkit.block;
+
+import org.bukkit.inventory.Inventory;
+import org.bukkit.inventory.InventoryHolder;
+
+/**
+ * Represents a captured state of a container block.
+ */
+public interface Container extends BlockState, InventoryHolder, Lockable {
+
+    /**
+     * Gets the inventory of the block represented by this block state.
+     * <p>
+     * If the block was changed to a different type in the meantime, the
+     * returned inventory might no longer be valid.
+     * <p>
+     * If this block state is not placed this will return the captured inventory
+     * snapshot instead.
+     *
+     * @return the inventory
+     */
+    @Override
+    Inventory getInventory();
+
+    /**
+     * Gets the captured inventory snapshot of this container.
+     * <p>
+     * The returned inventory is not linked to any block. Any modifications to
+     * the returned inventory will not be applied to the block represented by
+     * this block state up until {@link #update(boolean, boolean)} has been
+     * called.
+     *
+     * @return the captured inventory snapshot
+     */
+    Inventory getSnapshotInventory();
+}